it was rather sudden

It wasn't without precedent, though.

When he saved her from killing Rachel, he took a pretty big, impossible gamble, in terms of "how can I resolve this situation without compromising my cover?," (ditto when he told the guy who used to have a tail to keep his mouth shut, and not to mention his 'tude when he met Cal....).

IOW, he did show that his feelings for Sarah were strong enough that he kept stepping outside of his "duty" from almost the beginning, when she seduced him on the kitchen counter.

I think it's arguable that his devotion to doing "the right thing" was simply stronger than his love for either the Castor project or his love for Sarah, and that, when he knew he was dying, all of his love of life, all of the loves of his life became meaningless to his desire to vindicate himself and all of the bad decisions he'd had to make in the name of righteousness.