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).
Perhaps, but right now--as it stands--there is cartel activity associated with it. And funding violent cartels for any reason should be illegal, for the same reasons that funding any violent organization should be illegal.
The alternative of having a square job and funding the US military via compulsory tax payments kills and tortures an order of magnitude more people than the cartels, if you want to follow the money in this line of reasoning.
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.