I can see why some people doubt that Xykons use of Symbol of Insanity wasn't offensive, but I remember at least one published adventure in which a similar approach was used:
A Symbol of Death was hidden behind a curtain, and the enemies dropped it down once the players approached.
The idea (house rule) of limiting Symbols only on immobile objects has its merit, but keep in mind that this means you can't use symbols on treasure chests or books (read:
Ancient Tomes of Forbidden Magic and Spellbooks of Evil Archwizards), so I wouldn't use this idea.
Is the last surviving (non-paralysed) member of the Sapphire Guard committing
seppeku in the last panel?
Urk.
Okay, I can see why some people see the strip as more disturbing then funny.
But I an a fan of black humour, and the 7th and 8th panel are exactly that...
And I agree, one of the biggest rule/story issues is: where did Xykon get a bouncing ball? It doesn't have a price tag attached to it, so he didn't buy it in a shop (unlike his TeeVo or his Spyglass

). Probably has its own history attached to it.
The idea that the warriors are readying their actions to disrupt his spell casting makes a lot of sense. That might also be the reason why they don't attack in the panels 5-6 - they are readying to strike when Xykon casts a spell, but he doesn't.