...It's really difficult to know what to think about this, because neither of the given situations seem particularly likely.

The account of the guard who claimed to have shot him, Merda, is suspect considering the times and that the account doesn't quite match up with the angle of the wound.

On the other hand, it's unlikely that it was an attempted suicide because it seems uncharacteristic for one thing, but mostly because he was right-handed and the would was on the left side of his face.
...Then again, the inaccuracy could have been a result of using his left hand, which would be less precise...

And know, it wasn't very nice, but considering the violence with which all the Robespierristes were treated that day, it's hardly surprising.