The seven deadly sins are so named not because they are "the worst" sins (things like murder and theft aren't among them). The seven deadly sins represent emotional states which are DEADLY to the person who is under their sway.
Think of it in a Sun Tzu sort of way: if you give into sloth, envy, pride, gluttony, lust, greed, or avarice, you compromise yourself in a way which can lead to your death:
Gluttons could have heart attacks,
the lustful could get the deadly venereal disease du jour,
the greedy, envious, and proud could set a cycle into motion which would lead to their karmic undoing,
the slothful might oversleep an important doctor's appointment...
Which one do I like the least? Greed. Which is my favorite? Sloth, Lust, and Gluttony are all vying for my attention these days...