I don't care who they were, killing someone in their sleep is abominable.
The IDF should not act a judge, jury and executioner. Arrest and give a fair trial by all means, but to slaughter someone in their sleep is awful (and cowardly).
And yes, it's been accepted they were part of the PIJ, but have they been convicted of any offences that they specifically have committed? Do those offences carry the death penalty?