It is objectively ******* hilarious in hindsight, to be honest, but the reason people got mad are:
1. It's objectively a pretty mean spirited and brutal kill against someone who is ultimately a child.
2. The death logistically didn't even make sense with Ben's ability set. Alien X is acausal and wouldn't have been affected from being killed in the past, and the Sotobro Effect, a canonical aspect of Ben 10 regarding Time Travel, would have allowed him to know Hal was time travelling to the past and allowed AX to pursue him.
3. Ben's Omnitrix would have saved him from the arm slice in the first place, by either teleporting him, blocking the strike, or turning him into an Alien to prevent his arm being lopped off.
4. DB in the past has avoided brutally killing animated kids show characters in the past as to not potentially scare children who could watch the episode (it's why ever MLP death has been very tame, for instance), but yet Ben gets maimed and then splattered.
5. The controversy surrounding the verdict of the episode and the death being hilariously petty definitely inflamed people into disliking the death even more.
6. It's pretty anticlimatic in all honesty, Hal just stops the fight midway through right when AX gains the advantage and two shots a child.
Hope this helps explain to you WHY it was so disliked. They really would have been better off just having a massive display of uni busting or strength or something.