But he should be in a program that gets him help but also removes him from society at the very least.

Certainly, but not in prison. A secure hospital tasked with helping people like this. The United States prisons are some of the worst in the world.

His likelihood of reoffending is very high.

Then his stay at the aforementioned facility should be a long one.

Where do we draw a line?

For the long term, how about we fix our society so that people this don't happen - and that is possible. This offender is like this because our society made him that way. So, we are left with three choices. 1) Kill him outright. 2) Send him to prison for life. 3) Help him. Choice one is easiest - and cheapest. Choice 2 is more expensive and dangerous, someday, to society. Choice 3 is the most expensive but is the most compassionate.

There are absolutely people that are not able to be rehabilitated.

Wrong! This is a case of society doesn't want to rehabilitate anyone. It costs to much.

Do we allow them back into society at that point because they can't be helped?

But they can be helped. Society just doesn't want to. So, in the end it comes down to money. The bullet is the cheapest option.