I don't think it's "bad", OP, so much as it has some components that, taken in excess, can be unhealthful.
"In excess" is key here. A slice of deli turkey meat isn't going to poison anyone. A PP had a good explanation of nitrates; I'll also add that many of them are full of sodium - even if they don't taste salty, check the labels and you'll be shocked at how much sodium there is. Again, a little bit is fine, but keep it in moderation. Daily lunch is fine. Eating it for every single meal, too much for me.
I personally prefer fresh deli meat from the counter, where I can watch them slice it off the roast / turkey breast / ham / whatever. It just feels and tastes fresher than prepackaged meat to me. Even better is to carve it from my own roasted turkey or roast beef, but that takes a little more work than stopping by the deli counter

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