Gee if I was zevran on day two of joining the warden I'd slit his throat in the night go back to the crows have a beer and say mission accomplished!
And end up dead. This of course ignores the fact that numerous companions say they will be watching Zevran closely to make sure that he doesn't try to to assassinate the Warden. Unless we're saying that said companions are completely useless, and our Warden also isn't watching Zevran closely as well his chances at actually succeeding are about the same as the first time he attempted to kill you.
He is in the cage for going berserk and killing an innocent family including kids. We have zero idea what he feels are in his best interests when we meet him.
His best interests are not being in the cage, but he will still accept being left in it. Sten is actually more surprised that the Warden would actually WANT to recruit him.
You have to recruit them fairly quickly and that means you have to actually believe what they are telling you. Nothing at all suggests that any of these people should be believed in a thing they say.
By this logic none of the companions should be trusted because they are lying about everything they say. I find it amusing that you distrust the companions that are completely honest to you from the beginning. It's not like any of these companions are complete copies of Morinth who stands as the only companion in a Bioware game that is actually a threat to your character.
*Insert Lotion Soronarr's last quote here*
Mages may have many spells at their disposal, but seeing as Bioware didn't allow us to investigate the bodies we will never know for certain that he killed the templars. Rylock shows up and claims Anders killed the templars, but unless she is clairvoyant she has no way to tell that either and is probably using her own biases in her judgement.
Since you admit Anders has no reason to be violent towards you how is he a risk?
Is there a reason Anders would lie about his prior escape attempts? If a person has a violent history chances are they will continue to be violent in the future and vice versa. So his prior attempts actually do mean something.
Seeing as your Warden can protest joining the Wardens numerous times before Ostagar I don't see "being able to follow orders" to be a big requirement. We also have to take into account that the wardens have a history of recruiting violent murderers, thieves, blood mages, and other questionable characters, and to the best of our knowledge that hasn't backfired on them.
Edit: Yay for the quote system breaking.