How are plea deals moral?

I just watched a video of a guy being caught on camera carrying his wife's corpse. During the interview he lied and lied, said he wasn't there, was worried, didn't know where she was, etc.

They had fingerprints (from a garage that wasn't his own where he hid the body), him on camera carrying the body, found a weapon with his prints and had search results of him, before the murder, trying to find out how best to get rid of a body.

He went to court and eventually was offered a plea deal; for his guilty plea, they'd charge him with manslaughter.

It was pre-meditated murder, but everyone's just going to lie and pretend it was manslaughter to save a little money and time? Huh?