If the cops did indeed plant evidence then happy days for the defence since it should be easy to disprove. e.g. by simple handwriting analysis or other such means. But this is fantasy wishful thinking since he did write the words. So stick to the reality here

Handwriting analysis is hardly objective.

He shot the guy and confessed to it.

He’s pleaded not guilty, and unless you have more up to date information, he’s made no confession outside of the alleged note.