Mrs. Deagle believes that the gremlins are there to take her to Hell, which, in a way, might be exactly what they did.
When Gizmo gets wet, it produces more Mogwai. Unlike Gizmo, the Mogwais are mean and unruly. Assuming the reason why Gizmo maybe the only nice Mogwai is because he was raised to be a good creature. None of the other Mogwais were raised to be nice and cause chaos.
Another possibility: Maybe they're evil because one of the three rules were broken 'Never get them wet.'
Billy's Car Hates Him and after working perfectly when it really matters as things escalate, it then breaks down again. Since it was outside a gremlin-infested tavern when it broke down again, it might be that it stopped working not due to being a piece of crap but rather due to one of the gremlins meddling with it when the characters (and the camera) weren't looking at the right parts.
Of course, one does have to wonder why the gremlins would do that, since they seem to know what they're doing when they mess up technology in the films to get the result they want. Perhaps the gremlin thought making Billy's car suddenly work for him when they were taking over the town was funny enough to be worth essentially giving him and Gizmo an advantage.
Gizmo wilfully opening the store windows to stop Spike takes new heights when you remember sunlight kills both of them. It was only Gizmo's clumsiness that prevented it becoming a full-on Heroic Sacrifice.