In all honesty, I'd be kinda surprised if he targeted living allies at all with his A3 on auto...

He most definitely will target allies to kill on auto. I noticed this in my Minotaur runs with him (btw he will kill your solo'er and ruin your runs). You'll need to set his Ai. 

Now to what his Ai targets im unsure. I think it's almost definitely the leader position, but it could be something to do with ally HP as well. 

There's some interesting comps where you can put SWK in lead in arena and Valk will kill him to get Malice Unleashed.