My worst sins, IMO, are what I put in the garbage some times.
Do you know how many things your NOT supposed to put in the garbage "these days" ? Like batteries, "dead animals" like mice up to elephants I guess (why are animal based food scraps OK but a dead mouse or two isn't).
I don't even wanna say what else. So many of us IMO throw out in regular trash CFLs, tires, nasty chemicals, and all manner of things they legally or morally shouldn't. Regular trash is just so convenient and most locales don't inspect your garbage for violations; but some do.
On the plus side, I'll claim this is my worst sin.

Just an occasional battery, and probably some other stuff that's probably at least better than "medical waste" or "toxic waste"... :