Batman kills Venom by accident. Im sure the bat aint stupid enough to use a regular batarang, and looking at venoms skin type, he would throw a batrang, that would either electricute the person, like he uses on clayface, a freezing batarang to freeze him completely or one of his batarangs that has sonics, witch is alot. So Batman will discover Venoms weakness by accident then, beat him to the ground after he sees that it affects him. All his Batrangs that explode will irrate venom alot aswell.