In the realm of traditional RPG's the death of a PC (Player Character) is, in the very least, a notable event. For those particularly attached to their character, it can be a highly emotional event, particular for foolish enough to build a fictional manifestation of their corporeal self. With such considerations present, it brings the issue of guaranteeing player death in a given scenario. Whether it be a presentation of a neigh-invincible enemy force or the construction of a particularly sinister trap, the existence of certain annihilation is one that must be moderated with extreme tact and care. For the sake of simplifying this undoubtedly complex question of appropriateness, I will present the main arguments concerning this matter in a straightforward pro/con comparison:
The Benefits of Utilizing Instant-Death Scenarios
-Disposal of Annoying Players
A cheap, underhanded tactic, but sometimes a poisonous player needs to be separated from the group ASAP. A distinct lack in subtlety (especially in the case of a targeted death) makes it ideal for penetrating even the thickest of minds.
-Dispel Illusions of Grandeur (a.k.a. 'putting the fear back into them')
At higher levels of play, some hosts may find it quite difficult to instill the fear of death into their player's. Some, especially veterans of the game, may think they've seen everything; this is where an abrupt, unexpected death comes into play. Most adventuring groups should have the means to revive a fallen member at this point, so this typically traumatic event may come across as merely a current inconvenience to them.
-NIGHTMARE/GYGAX/PLAYER KILLER MODE
Though clearly a niche, certain groups relish the grim ultra-realism that is 'death at any possible second'. Such adventuring parties are almost exclusively well-seasoned players that can accept a PC death with reasonable maturity and may even play multiple characters at once (if allowed). Their play style is extremely cautious and well thought-out; reward this persistent foresight with non-compulsory deaths that are only granted during the rare lapse in judgment.
The Drawbacks of Utilizing Instant-Death Scenarios
-NIGHTMARE/GYGAX/PLAYER KILLER HOST
Simply enough, if you habitually deploy ruthless player-killing tactics, you will be known as a habitually ruthless player-killing host. Even the most fervently masochistic player will eventually tire of regularly composing new character records, thus enticing them to seek entertainment elsewhere. As for those even slightly attached for their characters? They'll avoid you at all costs, lest their special-snowflake concept fail to live an sufficiently long lifespan.
Any option that limits PC interaction is lazy, plain and simple, and forcing death on a player does exactly that; limit their options.
In the realm of random chance and probability, anything can happen. Despite knowing this, many hosts have been placed in an awkward situation after their deathtraps of choice have yielded an unexpected result. This can range from killing the wrong target/number of targets, to simply failing to dispatch anyone at all. Though this danger is present for most pre-planned factors, the chance for collateral damage is particularly great in the matter of instant death.
As previously stated, people can get pretty attached to their fictional creations. While some people can handle the death of said creation within a heroic or noble context, the idea of having their prized creation dispatched by a singular factor may often be more than they can personally handle. This can result in anything from a prolonged, awkward silence to a violent outburst. It's highly advised that you're familiar with your group's various temperaments and levels of maturity before making the decision to actively 'off' characters.
I've done my best to summarize the major points of this questionable conflict. What one does with the information I've provided is obviously theirs to determine.