Well, let us say, as our point of departure, that Anas al-Sharif celebrated the attacks of Oct. 7th while they were ongoing and thus he is a member of Hamas - and I'm going to set to one side for a minute whether or not I dispute that, and whether or not one follows the other.
Is that a death sentence? Are we really saying that writing the words "9 hours and the heroes are still roaming the country killing and capturing... God, God, how great you are" is a justification for killing a person? There is no account of the ethics of war - legal or discursive - under which a person becomes liable for expressing in words support for an act (even ones which are war crimes) within an IAC.
If your account of liability in war is so broad that you would include demonstrations of support as being grounds for liability, then all that any government ever needs to do to kill someone is make a plausible case that they were a supporter of terrorism. (And indeed, here we are, at the point where the wholesale destruction of Gaza finds its justification in the necessity to prevent and punish terrorism - this, as an aside, is an example of the doctrine of Kriegsraison, which was previously laid to rest by the judgements of the Nuremberg trials, but which has seen an operational re-emergence since the second Iraq war)
This is a very dangerous and slippery slope, wherein terrorism (defined, as it can be and is, unilaterally by states - and, as Chomsky notes, terrorism is usually used to describe violence which isn't approved of by the west, e.g. British proscription of PA) can be used as a justification to kill outside of the norms of liability in IAC.
Being a supporter of, or indeed even a member of, a proscribed organisation does not alone confer liability. And the reason for that should be obvious: were war to break out tomorrow between Palestine and the U.K., then the proscription of Palestine Action would allow the U.K. military to kill all of the people, mostly pensioners, who were arrested on Saturday for holding up signs expressing their support for the proscribed organisation.
The important point here is not whether he was a member of Hamas, or - as is being suggested here - spoke in support of the al-Qassam brigades (the militant wing of Hamas, bearing in mind that Hamas is primarily a civil and administrative body, not a military body). The important point is whether being a member of Hamas, or even speaking in support of the al-Qassam brigades, makes killing you allowable or justifiable under the rules of war. Alone, it does not. Not under any account of the ethics of war, legal or discursive.
Yes, three decades of scholarship in counter-insurgency and asymmetrical warfare has laid the groundwork for the killing of non-combatants who are members of terrorist groups. But even within the broadest frameworks, and here I would point to the reductive individualists (e.g. J. McMahan, H. Frowe) who reject the principle of separation (that the rules of war and the rules of the conduct in war are logically separated), mere membership of a group does not confer liability.