Like Squidward and Meg/Brian in Family Guy, Lars is the type who would willingly screw you over for his own ends, mainly because that's what being a teenager (a shitty one, anyway) is. He's not a jerk just to be a jerk or be rebellious. There's an endgame. That's probably why he gets the treatment he does. It'll stop when he drops that personal, which he won't unless there's a timeskip
You just described Ronaldo more than Lars in the bolded, other than lying to Sadie once and being kind of insensitive to steven once, Lars hasn't done anything too bad.
Ronaldo literally has engaged in "screwing [others] over for his own end" twice and both times it involved killing someone (steven and lars respectively). Not to mention that he was more than ready to kill a watermelon steven even tho it had shown sentience.