Why did Rose in the Titanic act as if others were guilty when she took a seat on a lifeboat as well?

I'm not 100% sure if it's only the fact that they didn't come back for the people in the water. It could also partly be a reference to survivor guilt (from Wikipedia):

Survivor guilt (or survivor's guilt; also called survivor syndrome or survivor's syndrome) is a mental condition that occurs when a person believes they have done something wrong by surviving a traumatic event when others did not, often feeling self-guilt.

Also, saying that they waited for an absolution didn't necessarily mean Rose herself thought those people were guilty although they might have felt guilty.

In the movie it's not clear if she blames herself. I would speculate that the fact that she really had to fight for survival gave her some kind of redemption in the end (these are after all somewhat irrational human emotions we are talking about).

Rose was convinced to take a seat in the lifeboat, she didn't intentionally displace someone (i.e. throw them out of the way), she was listening to Jack who told her it was the right thing to do. She was a woman, and so by "rights" she had a seat on a lifeboat as women and children are supposed to be the first saved.

Taking into the whole of Rose's character, it's likely she would have had survivor's guilt if she stayed in the lifeboat. She experienced it anyway when she survived on the makeshift raft (door) and Jack didn't.

This phrase doens't refer to the survivors. When she said that, Bruce Ismay is on the background (which was the Titanic's company director) and survived the disaster. In the following inquiries about the sinking, people never forgave him for having saved himself as opposed to give up his seat for women/children. His reputation was forever damaged and he never recovered from it, keeping a low profile afterwards. So the absolution never came.