So one of my townies have major beef with Gwen for like, no reason at all...he ran up to her on a community lot and started slapping her around...and his little icon was on fire and he had 100% negative social points with her. I don't know what the problem is, as she has never met him before and he did this while she was flirting with another sim.

I am thinking that because Gwen is such a little gossiper that perhaps she is spreading rumors about his wife and that made him mad? His wife slept with Gwen's room mate but why is he mad at Gwen?

Either way he is constantly kicking Gwen's trash can!!! It's so obnoxious, i had Gwen's boyfriend give him a little "chat"...but no matter what the guy sneaks around early in the morning and started kicking things. Why is he so mad?

Did she spy on him through the telescope? Did she ever have a date with him? make him selectable and check out his panel, what memories does he have.

It sounds like he caught his wife 'cheating' and is now angry with the whole household (of the sim his wife cheated with).

I could be wrong, but I would assume it's that- especially if he has a 'caught [sim] cheating!' memory.

Either that or he has a crush on the person she was flirting with at the time and thought she shouldn't have been flirting with them.

Sometimes relationship flags set or unset themselves for no readily apparent reason. Usually this involves sims losing their flags, but I had a townie once go ballistic on Tosha Go, in a full-on caught cheating event, even though she'd never seen the guy before in her life. But he couldn't have gotten a broken heart icon if he hadn't been set as in love with her.

Poor Gwen needs a restraining order, which you could probably simulate with the Simlogical prison token.

Maybe he is not nice?
My Mr No Nice Points attacks Sims for no reason at all - all the time.

from my understanding, sims always have a reason for getting angry; or for any behavior.
any behavior; most frequent reason might be personality.
getting angry; some possibilities::
-personality
-being spied on
-base game's Jealousy system
-Nightlife's Furious system
-possibly Apartment Life's Reputation system

I would say my no nice point sim gets angry due to his personality.

There's also aspiration failure, training, and relatives. The Grouchiest Man in Drama Acres has three more nice points than his oldest son, but his oldest son doesn't get into a tenth as many altercations, because his old man is the one seeing his girlfriends getting on about their lives. Once he got used to poking women he'd never met, it got to be a habit. He couldn't be trusted on the sales floor of his own business, and generally took his wife or youngest son to cover that while he cleaned the place and manned the register.

The headline made me laugh, as it reminds me of my 9-year-old when something breaks: "It happened for no reason!" :D

The maid in one of my families used to get furious with my sims for no apparent reason, resulting in her kicking their trashcan after finishing her shift. Which she of course had to clean up the next day.

If one of your sims is sloppy that's likely to be puddle rage. If one sim cleans up another's mess in the bathroom, they get angry at the one who made the mess. Which is hard on the maids.

In one of the patch notes they mentioned fixing a bug where a Sim would get pissed off at themselves for cleaning up a puddle that they made.