Killer weapons are shown to have a somewhat dark red color scheme with a unique, intimidating form (extremely sharp, curved blades, devil-horn like spikes pertruding from the hilt/end), and almost seem to be completely doused in blood. Fire Emblem: Binding Blade was the first game in the series to feature this visual style. Their high critical-hit rate makes them an extremely popular weapon type (especially when paired with Swordmasters, Berserkers, Assassins, or any other unit with an already outstanding critical-hit rate).
The main drawback of these rare weapons is that they are expensive, barely below the cost of Silver weapons, and their durability is fairly poor (usually 20 or 30). They cannot be forged in most games (the exceptions being Fire Emblem: Shadow Dragon and Fire Emblem: Awakening).
Killer weapons can be more dangerous than Silver weapons when used by enemy units. This is due to the increased critical rate, whereby it may result in a 1-hit kill if the circumstances allow for a critical hit depending on the character's stats.