I don't think Jisai had much effect on him at all. If you look at Kojiro from when Jisai met him to now...there really isn't much difference. Kojiro's backstory is Jinsai trying to be some diabolical mastermind and Kojiro just shrugging his shoulders and wrecking his plans.
Also...Jisai still has both of his thumbs in that 10 years ago page.
Oh yeah, I can agree that Jisai does gives himself too much credit, but I don't think Kojiro would have become a killer without his actions.
Yes, I can see that Kojiro would have been "liberal" with his use of violence, I don't think it would have become the same without Kojiro's "education".
His "command" given by his mother could lead him to hurt people, but I bet that he would have seen that living peacefully was an option too, a better one even, but now... I think the two "commands" have fused too well and he is the "machine" Jisai claims him to be.
And yeah, I noticed the thumb, but the moment he took Kojiro in I think it was before he fight with Ittosai, but I admit I stopped noticing if after he established the Brood his thumb was intact or not.