Presidents can already pardon people who murder their political opponents. Famously, the Reconstruction-era presidents pardonned every single confederate so as not to divide the country further and increase tensions.

You are clutching pearls over something that is already allowed. Like I said, Congress can impeach a president who does this if they don't like it.

Moreover, a president can't pardon state level offenses anyway, and I would imagine the murder would have to take place in a state. The state could simply retry the case. States have much more discretion in the sorts of things they can criminalize.