User blog comment:Master Sima Yi/Assassinews 03-03-'13 — Assassin's Creed IV: Black Flag trailer and screenshots/@comment-5542440-20130405084710/@comment-1888015-20130415044513

If you've read Forsaken, you'll understand why Haytham was so ruthless as he was in those instances.

But, for those who don't know, Haytham's friend Jim Holden had been captured as he stayed behind in Damascus so that Haytham and his half-sister Jenny could escape, and Holden was castrated because of it.

Haytham basically saw how callous and brutal people can be just to fit in with their beliefs, and so he just didn't see the point in trying to tiptoe around issues when he could just take the most direct path and get the job done. That, and considering how much betrayal he had faced in his life, he felt the need to be selfish to take what he wanted in life, instead of letting it screw him over.