Does Howard abduct deliberately Michelle or is he trying save her life?

I'm pretty sure it was just Howard was going to kidnap her even if there wasn't an alien attack. There just happened to be one right when he was going to kidnap her.

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I actually believe Howard's account of the events.

I believe that he was racing back to the bunker and accidentally ran Michelle off of the road. I think he saw her in the wreck, and out of a mixture of feeling bad about hurting her and wanting a replacement daughter, took her home with him. He probably saw it as a move that was good for both of them. I feel like if he was going to stalk someone and replace his daughter, he'd pick someone who was a little closer to the daughter's age--younger than Michelle, who is clearly in her mid or late 20s.

I think that the fact that he has a bunch of "daughter" stuff in the bunker (like the magazines and the clothing) is a detail that you could read either way. The innocent interpretation is that he likes having his child's things around to remind him of her. The more sinister interpretation is that he was planning/hoping to abduct another woman as a replacement for his daughter.

I think that one of the reasons that he gets so riled up about her not being "grateful" is that he doesn't want to admit to himself that he isn't just this nice guy showing her hospitality and mercy. He took her partly for his own selfish reasons, which is creepy.

Well, we can conclude that Howard was the "bad guy" because he killed two people (Emmett and Megan). He was quite dangerous and mentally disturbed. So, it's ALMOST FOR SURE that he kidnapped Michelle for his own (strange) reasons and COINCIDENTALLY there was this nuclear war/alien attack. He didn't want to save her life in the first place, that's obvious. The alien attack was his excuse to retain her in the bunker, with him. Methinks, the main reason he killed Emmett was that Howard was jealous of him, as you can see in the previous scenes.

But, in general, I think the director wanted to adress the following issue:
What would you do in a similar situation:

The dangers of what you know (Howard) VERSUS the dangers of the unknown (the outside world).

Michelle finally did the right thing, taking many risks.

Cesarat37 - he didn't kill Megan (his daughter).... the daughter and mom left him and went to Chicago when he showed enough crazy to want out (I think just because he was a bunker conspiracy nut).

He obviously kidnapped the other girl (green earring) and pretended that was his daughter. We don't know her fate - I suspect she managed to escape briefly and he killed her. Why else would he have a vat of acid in his bunker if he didn't think he'd need to cleanly get rid of bodies? I think he killed before, and thought of it as a good solution to have 'just in case' before shutting the bunker up for good for a while.

The film doesn't make it clear.

Perhaps he was planning on abducting Michelle and rammed her car in order to do just that and it was just a coincidence that there was an attack at the same time.

On the other hand, he could have been telling the truth, that he'd heard about the attack and was racing home and recklessly caused the accident. At that point, he saw that he had an opportunity to replace the girl he'd previously killed and so "rescued" Michelle. As he made clear more than once, generosity was a thing of the past, so if he was telling the truth about the accident, why would he save her?

Either way, I'd call it an abduction. Either he went out looking for a woman to take or it was a crime of opportunity. Regardless, once she was in the bunker, he expected her to "behave" (his word) as he wanted her to.

Maybe it's just my own sense of logic, but between:

1) Howard panicking because of attack and seizing on the chance to abduct Michelle

2) Howard abducting Michelle and by coincidence an attack happens at the same time

scenario (1) somehow seems to make more sense to me and seem more plausible.

I do agree with others that even if the attack had not happened, Howard was clearly gearing up to abduct another girl/woman.

We actually do have evidence that he abducted her and that he intentionally rammed her. He said he was driving recklessly "on his way home"...except she was driving northbound, away from where he lived. If he had been telling the truth, he would have been going southbound. They were going the same direction - he came from behind and around the side. Thus, he was lying. Also, he saw her at the gas station, so he already knew what she looked like and presumably targeted her.

There is so much speculation about whether he was a bad guy...I really wish more people had noticed those details because they change a LOT about our conclusions. I appreciate that it was subtle, but perhaps it was a bit too subtle.