To destabilize Mexico, resulting in more immigrants. He also wanted to blame US guns for Mexico's problems.
I believe it was for multiple reasons too. The last one you give, "He also wanted to blame US guns for Mexico's problems.", is a reason with ties back to Obama's abundant efforts to take our guns from his very first days in office.
WHY?
Obama doesn't care about the well being of others. That's obvious from the desolation his policies have created. So, what could be the real reason for taking American guns?