It sure is a sin
I'm not convinced it is.
For one thing, Jesus said No one takes it from me, but I lay it down of my own accord.
this is clearly a statement of intent to die as well as taking responsibility/accountability thereof.
Moreover, Samson is listed in Hebrews's hall of heroes
when his last act was suicide, a suicide which God honored: And Samson said, Let me die with the Philistines. And he bowed himself with all his might; and the house fell upon the lords, and upon all the people that were therein. So the dead which he slew at his death were more than they which he slew in his life.(Judges 16:30)
To say that God honors sin is disgusting. Yet it is clear that he committed suicide on our behalf in the person of Jesus, because our sins demand death. He purposefully, intentionally paid that cost, by Himself and of Himself; and thank God that He did, otherwise we would be utterly doomed.