Ya, Leliana really jumped the gun on that part. My Inquisitor will be upset if she does that during my human mage canon playthrough. "We were talking, we were getting more information, then you just HAD TO UPSET HIM, NEEDLESSLY, and forced a conflict."
I wouldn't be friends with him either, as his group is responsible for some of this mess, but I would accept them as underlings/subordinates/agents after imprisoning Alexius. Then would either gather info from Alexius and make him an underling overtime or send him back to Tevinter if they request him.
Just some thoughts after seeing that scene.
Still, the Venatori either get with the program and be absorbed into the Inquisition or perish. No way their organization is doing any good, but the individuals may redeem themselves.