Your friend would be furious if you promised you would water his plants while he was out of town, and then let his favorite fern die. To die is to expire, or to cease to be alive.
The verb to die means "to stop living," and it's also used to exaggerate a need or want, like when you say, "I would die to have those shoes." When die is a noun, it means a metal-cutting tool or the single form of dice: "Hand me the die, it's my turn to roll." When you're talking about permanently staining or coloring something, you have to use the word dye.