Whether it is unfair or not depends only on what the players expect the game to be like. If your players know that the world is full of lethal loaded guns in the form of volatile magical objects, it is not really unfair - they'll expect expect warnings to be taken seriously and will probably do so. This style of play places a lot of emphasis on players making good calls, as opposed to the character knowing how to avoid such dangers.
However, Dungeons and Dragons is usually not played like that, for several reasons. Creating a new character from scratch to replace the dead one can create a nasty pause in the game, scrap hours of work spent on building the last character and is usually off-putting narratively. DnD borrows heavily from works of high fantasy adventure such as The Lord of the Rings, where important good characters die only when performing heroic feats, never because they made an obviously frivolous decision with a dangerous magical object just to see whether they'd really get blasted to Atom Bombadils. Care should always be exercised when subverting the conventions of the genre you're playing.
Before introducing the object, consider all the implications of what you're doing. It's not just a single character death you could be dealing here. If your players are not used to characters dying easily, your little trap can change their perception on the game entirely - you're giving them the message that characters can die easily and at any time, as a result of a single silly move. If you don't want your game to become one where the players proceed with paranoia at all times, change the item.
You can also keep the item as-is, and when the player asks to use the item in a dangerous way, be frank: "If you do that, your character dies. Are you sure you want to do this?" This way your players will have a fully-informed decision to make, and you will avoid souring the mood just because someone made a silly decision.