Is it permissible to eat "Ayyash" fish that is struck on the head and then decapitated while still alive? And does this contradict "Ihsan al-Qatlah" (kindness in slaughter) and the retention of blood in its body?

First: It is forbidden to kill fish by cutting them while they are still alive, as this constitutes torture. This is indicated by the hadith: "Indeed, Allah has prescribed excellence in everything. So, if you kill, then kill well; and if you slaughter, then slaughter well. And let each one of you sharpen his blade and relieve his animal." It is preferable to leave the fish to exit the water and die, or to slaughter it with a knife if it is large.

Second: Eating fish after it has been cut is permissible, because its lawfulness does not require ritual slaughter (dhakāt). The Prophet, peace and blessings be upon him, said: "Two dead animals and two types of blood have been made lawful for us. As for the two dead animals, they are fish and locusts." The Ummah has reached a consensus on the permissibility of fish and locusts without ritual slaughter.