There is irony as well as pathos in these lines, one proceeding from the other. Lancelot is flashing in all his armour as he travels down the river, inspiring her to free herself from the spell that keeps her in the tower. She wants what he represents:  life, vitality, and freedom--and romance, no doubt as well, for the narrator depicts him nothing if not a romantic figure from that romantic legend. He, however, cannot be blamed for her demise; indeed, he doesn't know her and doesn't know what brought her to that spot. That is Tennyson's point:  the poem sympathizes with the plight of the Victorian woman imprisoned in a tower above the rest of society. Lancelot's works evoke pathos because any  woman who tries to break out of her prescribed role--any woman who tries to break free of gender ideologies, which is what that tower and spell symbolize, is doomed to a figurative death. Victorian women needed grace to endure--that too is part of the ideology of the time.  And how can we not feel sorry for her--and him--for he hasn't a clue.

Pathos - the quality in something which arouses pity, sorrow, sympathy, etc. (thanks, Mr. Webster!) - is evident in Lancelot's final words because he is so dismissive of this beautiful woman. She has sacrificed her life to see Camelot, and him in particular, and all he can say is, "Hmmm...she was pretty, I hope God grants her grace."  One feels sorry for the Lady of Shalott, and even a bit sorry for Lancelot that he is so thick-skulled!

Check the link below for more information on this beautiful poem!  Good luck!