If Palpatine had wielded lightsabers taken from slain Jedi – using Jedi weapons to kill Jedi as a sign of sacrilegious disrespect, not because he actually needs to resort to using them – I wouldn’t have had as much problem with him wielding such weapons. For example, if he’d whipped out Windu’s saber when going to fight Yoda, I’d have been totally fine with that (provided the following duel wasn’t as horrendously executed as it was in ROTS).
As it is, having him carrying around a lightsaber of his own design as a default form of offense/defense is antithetical to everything Palpatine represents in ROTJ.