Popa, I think most frum vegetarians or near-vegetarians are not worried about the slaughtering as much as what happens before — extreme cruelty on an unprecedented scale, documented ad naseum by various kinds of evidence (including video — try watching the video “farm to fridge: the truth behind meat production”). Some are concerned with “shackle and hoist” — shechting animals upside down — but again this is right before shechita, not the pain from the shechita itself.
But it turns out that some authorities do actually consider the pain involved in shechita to qualify as tzaar baalei chaim. To quote from R’ Sears’ book again: