I'm a spearo and I don't know the facts in this picture but I'll try to point some things out.
*) This picture is a couple of years old, I think I first saw it the beginning of 2020.
*) A trigger mech is typically located at the end of the shaft and the victim's arm would probably not be long enough and be able to pull the trigger so that probably rules out "Self-Inflicted". So someone else probably pulled the trigger.
*) You are not allowed to spearfish in Europe/Turkey on scuba, you must be freediving with no tanks. And lots of freedivers dive one up and one down. Some(?) spearo's return to the boat with loaded guns to save time (yaa I know it's not safe-I don't)
*)The angle of the spear in the picture suggests that it was fired from someone 'above' the victim
Now comes the pure {speculation} on my part based on my spearfishing experience.....................
The victim was returning to the boat with his gun still loaded. He handed his gun up 'butt first' so the spear would not be pointed at the deck person because it was still loaded. The deck person grabbed the speargun by the handle trigger and accidentally squeezed/touched the trigger firing the spear. If you picture the victim being in the water still and handing his gun up to the deck person, the alignment in the picture is almost perfect for what would result & happen. {/speculation}
PS>> Not all of us spearo's are bad people, most are very very safe and fun to dive with.