Because they are humans too, not a faceless monster or boogeyman. Further, while everyone can agree that Nazis are bad, not all anti communists are that rabid, and the vast majority can be rehabilitated. They deserve a chance at life too.

They were tricked, lied to, brainwashed, kept ignorant, and so on. That can be fixed. Not in all scenarios, but many.

Also killing people directly or indirectly is extremely traumatizing to the human brain, and causes long term and unavoidable damage. It’s best to avoid that when unnecessary.