I totally get the instinct to condemn the attack, since it's truly, deeply viscerally horrifying (not to mention terrifying!), but most of the rules about how you're supposed to conduct war basically boil down to 1. Make a reasonable effort to avoid disproportionately harming civilians 2. Don't go out of your way to inflict pain and suffering on your enemy beyond what's a necessary part of trying to kill or neutralize them 3. If your enemy is completely at your mercy, you have an extra duty to uphold 1 and 2.
Again, the pager attack is new, unusual, and just very upsetting. But it harmed civilians at a remarkably low rate, and the method of harm wasn't meaningfully more painful than just shooting someone. It compares very favorably with just bombing people on every metric other than maybe how scary it is if you're a combatant.