How bad is it to drink alcohol on wellbutrin (bupropion) is it guarenteed to make you more depressed? I already know about the seizure risk.

Not. Guaranteed but probable. Alcohol second phase is a strong neuron-depressant, so somebody with depression may get worse.
Not a good idea. It is not guaranteed to occur. Per Drugs.Com Drug ineteraction Checker-
“Using bupropion with alcohol may increase the risk of uncommon side effects such as seizures, hallucinations, delusions, paranoia, mood and behavioral changes, depression, suicidal thoughts, anxiety, and panic attacks.” There is moderate risk. Ref: https://www.drugs.com/interactions-check.php?drug_list=1034-14582,440-203.