ugh , yeah. can't argue with that.
but there also case where they don't care about the line. especially when they have backing (like high position parents or relatives, money, etc)
That is still a line. Even if they have backing be it high position family, money or etc. The line is simply bigger for them and with experience they know how much they can get away with. That isn't to say bullies don't cross the line at spur of the moment, but that usually is when they get in trouble. But many bullies know their line and stick within it. Not so for victims who lash out without thinking (unless they are lucky)
To give you a personal example, there was this kid who was semi bullying me from time to time. Mostly because there were easier targets in class and he probably knew I was stronger than him. But I tended to be very tolerant so sometimes he took advantage of that.
So one time in class during a free class, he came up, took my paper and crumpled it up. Normally I would just get annoyed and let it go. But that day something snapped and I came up to him, lifted him by the neck with one hand and flung him. He crashed into multiple desks and fell on the floor. At that point, I woke up from snapping seeing everyone including the teacher staring. I was like "oh shit, I'm going to get suspended". That is an example of a victim losing control and picking a poor location while going too far and crossing the line. If I at least found him outside of school and did that, it would have been easier to get away with then doing it right in middle of a class
So everyone is staring there frozen. But the teacher comes to the kid on the floor and tells him to go to the dean. Yes, he got in trouble. It seems the teacher happened to be looking down and didn't see me pick him up and fling him. All they saw was end result after the crash noise. And since he fell quite a bit away from me, the teacher likely thought he was fooling around and ended up toupling over the desks. This is an example of perception. What you see and what others see are 2 different things. Someone can punch you, and you punch them back but no one saw you get punched, only you punching them. Bullies also know how to abuse perception unlike a person lashing out. I just got lucky
Of course the class always follows the rules of "snitches get stitches". So nobody said anything. Even the bullies so called friends said nothing and just laughed at the odd turn of events