So Marty was completely able to defend himself..

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apparently that's how it really happened, too, even in the book that scene is described just like that, and there were other similiar instances.
It is not the solitary occasion where Marty began to round on him.

Marty never really had any real paralyzing fear of fighting Bobby outright..he was clearly not like "ok, I cannot stand up to this guy because he could totally take me and beat the shyt out of me"...it was not like that.
Probably they are evenly matched anyway, and -Bobby-- never presses the issue when he meets stiffening resistance.He never allows it to escalate to a potential all-out showdown.

You could offer more than one theory why not. It's been discussed elsewhere on the board.

As for the killing, why not just himself personally beat Bobbys asse, hell, why even bother force a fight, why not just simply tell the psycho to FCKU OFF..check-mate.
Why all these peripheral approaches to ridding himself of Bobby...first, pressing his parents to move to another county or state, when that does not come off, the plan to actually murder him.

Why? well, that's the interesting question, psychologically..

They were friends for years and as anyone knows, in Childhood and Teenage years, it feels like a lifetime. The psychological abuse is what destroyed Marty. It's like ANY abusive relationship, things are tolerated when they would never be in any other situation. Add in the fact that these are KIDS, it makes sense. It's plausible.

I think it was Marty's girlfriend's idea. It's been a long time since I read the book but that seems to be what I recall.

Exactly. There were so many ways he could walk away from Bobby. He just got so used to the violence from their steroid abuse.