If he was funding cartel activity by purchasing meth from Mexico, you can be sure that harm resulted (even before you get to the destruction that meth addiction causes).
Furthermore, the destruction caused by the individual use of meth cannot reasonably be attributed to anyone other than the person who willingly purchased, acquired, and ingested the substance with full consent.
It's not moral to ask citizens to pay taxes, but it's not immoral to pay taxes: you do it because it's the least harmful option available to you.