John Paul Mugabi is a sweet enough fellow, although at work he has been known to behave like something you might find at the bottom of your trash barrel, eating bent-up soup cans and smelling around for old meat. They started calling him "The Beast" because he fights like one.

You only imagine him growling and drooling over his poor, toppled opponent. What he really does is hurry to the neutral corner and suck his teeth, looking to savor the fruits of his work.

Mugabi, from Uganda, is the 26-0 terror who will challenge Marvelous Marvin Hagler Monday night (the bouts will be on closed-circuit television at Capital Centre) for the undisputed middleweight championship of the world. He speaks very little English and sometimes does not speak at all. One thing Mugabi likes to say is, "Ah will nuke 'im out, okay?" Another is, "He gonna feast on the beast, yah!"

There is another sweet enough fellow who will be featured on the card, this one against Thomas Hearns, the World Boxing Council super welterweight champ now fighting in the 160-pound middleweight division. James Shuler, who will be fighting for the North American Boxing Federation championship, sometimes goes by "Black Gold," but it is best not to talk to him or even be found in his company in the days before a big fight. Shuler, who is 22-0, gets mean and, as he explained it, "I get all full of hate. I can't stand to be around certain people, especially those close to me.

"Already I hate Tommy Hearns' camp -- his friends, his family, him. I just get in that state of mind. Even my body goes ugly when it's time to fight. I hate my own people -- my brother and my cousins, everybody. By the time the night of the fight rolls around, I'm even more vicious. I hate all human beings. I want to kill somebody. I want to kill the man in the ring, just tear him apart. What's weird about all this is that I'm a believer in Jesus Christ and it's like I've all of a sudden been zapped by the devil and gone crazy."

Both Mugabi and Shuler are gifted fighters enjoying careers that place them as top contenders in the middleweight division. Perhaps for their own sake, it is too bad they are not fighting each other. Hagler, at 61-2-2, need not shave his head to come across as the sort with a monstrous mad on for the world; he continues to fight with the desperation of a man possessed. Hearns is 40-2, a native of Detroit who likes giving angry stares to strangers and letting his upper run of yellow teeth ride over his lower lip.

Hagler and Hearns are men unaccustomed to fighting men who have never lost.

Said Mugabi, "I'm gonna do my fight because I always do my fight. This is how it is. If he runs like a cat, I will run like a dog."

"Thomas Hearns is in my way," Shuler said. "He's been a great champion, a great athlete, a great person. I don't really hate him personally, but I hate the man I'll be getting in the ring with on Monday. I'm starving to win, I've been waiting so long. Nothing will stop me."

Mugabi has won all of his fights by knockout. But what is even more amazing is that his 26 bouts have lasted only 73 rounds, total. Ten ended in the first round, six in the second.

His longest night of work thus far has been against James (Hard Rock) Green, who went more than nine rounds with "The Beast." Thumbed by Green late in the second round, Mugabi fought virtually blinded in the third and endured a terrible beating. He roared back in the fourth, became angry in the fifth and ended it in the 10th, making Green the only fighter ever to go more that six rounds with "The Beast."

Asked about Green, Mugabi said, "Oh, that fight, not so long. He thumbed me so I nuke 'im down. It was a short fight. I disliked him."

As an amateur, Mugabi won 106 of 110 bouts and fought as a welterweight in the 1980 Olympic Games in Moscow, breezing through the competition before meeting a Cuban in the finals. When people say John Mugabi lost to Andres Aldama and took the silver medal, they say he "lost" the fight and give you a speech on the no-count Red menace. "I think the communists ganged up on me a bit for that decision," Mugabi once said. "I was fighting a communist in a communist country in an Olympics that was boycotted by many western nations. My only chance was a knockout, I think, and I didn't get it."

Promoter-manager Mickey Duff, a Londoner with a hawk's eye for talent, watched several of Mugabi's Olympic bouts on television and decided he wanted to work with the young man. Duff had handled 13 world champions and almost singlehandedly controlled boxing in Great Britain during much of the 1970s. Duff later had to settle on a compromise with West German promoter Wilfred Sauerland, who enlisted Mugabi first. Duff and Sauerland still share Mugabi's promotional rights.

"The Beast is the best all-around fighter I've been associated with in 40 years," Duff said. "He is, without a doubt, the hardest hitter I've ever seen."

Unlike Mugabi, who is heavy up top with muscle and a big-bang puncher, Shuler is tall and lean and a fighter of finely honed tactical skills. The North American Boxing Federation middleweight champ, Shuler's last four bouts went the distance, but all but two of his previous 18 fights ended by knockout. He and Hearns have known each other and been friends since they were boys of 10 and 11. One day this week, Hearns approached Shuler as his old friend joked with reporters and said, "Don't lie, James. Now, don't lie. Tell these men the truth, James."

Shuler offered a lovely smile but quickly broke out in a sweat that sent little glistening beads dribbling down his cheeks. He laughed and performed for those gathered, but he apparently was shaken. Hearns often boasts of being able to turn his rage on and off like a light switch. Shuler's switch, at this late date, remained in its state of perpetual fire. How do you love someone and hate someone at the same time? he may have asked himself.

"When I hurt," Shuler said, "I know I'm getting right. I've been hurting. This fight is making me hurt in a way that I haven't for a long time."

Shuler grew up in Philadelphia, in a neighborhood of housing projects about five blocks away from Joe Frazier's gym. One day after swimming practice, he and his brother stopped off at the gym and asked the former heavyweight champ to autograph a couple of black-and-white glossies. Frazier signed the photographs and told them to round up some shorts and a T-shirt and come back the next day. The two Shuler boys soon became regulars.

"My mother was against it at first," James Shuler recalled, "but we thought it was fun. My brother won a trophy and we all wanted one. I had my first fight when I was 10 and I won a trophy. My mom still had trouble handling it. Whenever I fought, she would hide in the ladies' room until it was over and someone came to get her."

Shuler was a member of the U.S. Olympic team that boycotted the Moscow Games in 1980. He had hoped to built a reputation similar to that of Sugar Ray Leonard and return to this country and million-dollar paydays. Shuler, often prone to exaggeration, said he lost about "$16 million, maybe more," as a result of the boycott.

"I'm so hungry to win," he said, "and so ready to fight, I'm starting to get afraid of myself. Even if my left arm fell off, I'd still go in there and get Tommy Hearns with my right."