Why was Kuwait slaynt drilling Iraqi OIL?

Slant drilling was a way of stealing--so to speak--Iraq's oil. By violating the drilling agreement, Kuwait could get more of the pool of oil, thus earning them more money at Iraq's cost. This alone was a nice incentive, but additionally, Iraq owed Kuwait a huge amount of money after the Iraq-Iran war. If they could hurt Iraq's financial situation, it could be used as leverage to get something else (land, political position, etc.) better than money. (Kuwait wasn't exactly hurting for cash.)