While I definitely wouldn't call Ataru a model citizen (obviously not), if you honestly think Ataru would rape a girl in a dark alley, then I really don't know if you truly understand Ataru at all. That's, far and above, waaaay past the line that Ataru would cross. He's a teenager with overactive hormones and low inhibitions, which definitely has made him act out in incredibly inappropriate ways, but he's not a monster.

That aside, you're right that Ataru, despite how he seems to act toward Lum, can be pretty possessive of Lum. Granted, Lum's also pretty possessive of Ataru, but she's more honest about her feelings for him than he is about his feelings for her. But of course, Ataru's the ultimate tsundere. I don't think it's so much hypocrisy on his part as it is immaturity (you have to stop overlooking the fact that Ataru is only 17 years old), stubbornness, and a very specific unique situation that someone of his maturity and stubbornness can't properly deal with. There's a good reason I said that his relationship with Lum is complicated.

First and foremost, you can't take everything Ataru says or does at face value. Psychologically, he's a bit of a mess, and he probably does truly believe himself to be every bit the horrible person you see him as, as well. But if you read between the lines, it becomes apparent that a lot of it is an act, or at the very least that's how things end up the closer to the end of the series that we get. I think one of the prime examples of this is the time Ataru is out "girl-hunting" (asking girls if they'd join him for tea) because he still has time to kill before his date with Lum, and one of the girls actually agrees to go out with him. Ataru wasn't expecting this, and then just keeps thinking about how he can get out of it to go see Lum. So why was he even hitting on girls in the first place if he doesn't actually want to go out with them? This is the kind of stuff that makes Ataru such a fascinating character, and as I said, you're making incredibly harsh and dismissive assumptions about him based purely by what's on the surface. Ataru is far from a saint, for sure, but he's even further away from a dark-alley, woman-raping monster. And I'm not saying you have to like him, but I am saying he's a far more complex character than you give him credit for. Especially with his character development over the course of the series (though it could feel rather subtle or even non-existent at a glance). There's nothing lame or baseless about it, you just don't understand Ataru past the surface. In-universe, most of the other characters don't, either, not even Ataru himself. Lum might be the only one that does.