Many folks have committed suicide by getting themselves murdered. We have nuts, martyrs and hopeless folks. There are therefore exceptions to your "universal", therefore its not a universal. Sorry.
Actually, there are many situations that could arise that are better examples than you've given. A parent could volunteer to be murdered in place of a child. These are the periphery. Parents, martyrs, hopelessly ill all have ulterior motives. I'm talking simple murder, no special "72 virgins" reward as motive. Nothing, you're just dead. Mentally ill don't count, they don't have the proper capacity for decision making.