Did George Milton have to kill Lennie Small?

I think it’s one of those plot points that the author leaves to your imagination.

I think George finally came to the conclusion that Lennie was an unguided missile, and that (more) devastation was inevitable.
Turning him in to the authorities would mean unbearable suffering for Lennie.
This all from George’s viewpoint.

Wasn’t Lennie becoming dangerous?

@Dutchess_III He accidentally kills Curley’s wife after she was trying to, ummm, get a little too friendly with him.

I thought it was out of love, sparing him future misery: really the only option he had. But it has been a long time, decades.

Yes. This is a core part of the novel.
Lennie was either gonna get killed by the other men or George. The other men were only doing it because they hated Lennie, George would have done it for the sake of Lennie’s safety and mental state. George would have felt guilty if he did not handle it himself.

As an act of mercy, yes.