"If he’ll kill himself, then he’ll kill us."

With difficulty.

If he is willing and able to kill himself, he would certainly be able to kill us.

Bit odd though as you wouldn't know if the first part is true until someone actually does kill himself, in which case he couldn't then go on to kill you. The first part is trying to 'prove' the 2nd part, but it can't.

How about..

A: Look, say something. If he cuts that rope, he'll kill himself.

B: If he'll kill himself, then he will kill us. The rope is also to attached to this platform.

B: If he'll kill himself, then he will kill us. Not logic. He will be too dead by then.

too dead? I don't think too can modify "dead". am I wrong?

You're right. You can't really modify dead. There is the movie The Princess Bride where that is debated in a very funny scene - "mostly dead is not all dead" - but it's done for humor.

I think that in the situation described, they are saying that even if he doesn't care about his own life (if he cuts that rope, he'll die), does he realize that his actions would also result in our

Anonymoustoo dead? I don't think too can modify "dead". am I wrong?

I've used it in humor, as

Logical:
he kills us, then he kills himself
or
he kills himself and us at the same time (say a blast)

Illogical (in my view):
he kills himself, then he kills us (he would be dead by then)

Perhaps some people reading this are confusing the "then" used in an if-then statement of logic with the "then" used to show a sequence of time.

I sat down, then I took off my shoes - sequence of time.

If it is Tuesday, then I have choir practice - logic.

OK, got you, GG.
Still, the original sentence is a bit confusing on this basis.

<he kills himself, then he kills us (he would be dead by then)>

"Then" has the meaning of "as a consequence" there.

he kills us = as a consequence, we also get killed