Why would a lawnmower die when blade is engaged?

Most of the time this is caused by a bad seat safety switch. Under or along the hinge area of the seat you will find a plug. It can operate two ways. If it's normally closed, unplug it and try to engage the blades. If it's normally open, unplug it and put a jumper wire between the two contacts and then try to engage the blades. If either of these two work then you need to replace the switch.