He rescued her from the crash site, if not, she probably would have been captured by the zombies and eaten.
Doubtful. Isaac appeared on the crash scene relatively quickly and encounter no cannibals until returning back to the children.
When she appears injured he immediately goes out to get medical supplies she requests.
"immediately" No. She asked for the supplies and he says a bandage will do. Not until Claire says she might die AND THEN HE'D BE ALONE does he leave. This action by Claire not only gives us insight into her captor, but into her. She already believes at this point she is absolutely being confined for selfish reasons, and will have that suspicion further confirmed by the fact you leave out; that he knowingly removed her communicator from her unconscious body. When she asks where it is she gets no reply. This is no benevolent savior. At best this is a lonely nutcase who didn't want this uninfected woman to be rescued by her people or rejoin her children, at risk of becoming alone again. At worst, this is an evil man who has far more sinister desires for the uninfected woman he found.
And for those who will cite him locking her in the room, it’s possible he did so because he knew she would return to the shuttle and believed she would be caught by the cannibals, killed and eaten.
And what would give him the right to imprison her, even if he believed this? He made it very clear he had weapons, why not escort her if he's so benevolent and so concerned? He was certainly capable enough to make it to the shuttle again. Because he is a kidnapper who didn't want to risk losing his catch.
No matter what you might believe about his intentions, he may have just been protecting her the best he could.
Everything he said and did suggested
That murder was unnecessary, she had a large knife and gun she could have just pointed it at him and left the building.
She didn't have the gun at first, she got the gun from stabbing him. I don't know how anyone could think a mother is going to risk a chance to escape her captor and save her endangered children on the chance that this clearly mentally unstable and physically more powerful individual might not overpower her if she doesn't surprise him.