Kill

A Kill occurs when a champion decreases another champion's health to zero when not having any abilities remaining which prevent death. The last player to strike the target champion is awarded with the kill, and all contributors within the last 10 seconds are awarded with an Assist. When a champion is killed, the player who scored the final hit receives up to 500 gold. Killing the last living member of a team is referred to as an Ace, which is also announced. Should an enemy turret kill a champion without said champion being hit by an opposing champion within the last ten seconds, the death of the champion is referred to as an Execution. The same situation applies to a champion being killed by a minion or a neutral monster. An execution offers no gold or experience. If the champion is killed by a turret, but was damaged by an enemy champion in the last ten seconds, the kill is awarded to the enemy champion.

The Amount of Gold
Each death of one champion reduces the gained rewards by 15% down to 15 gold. If you break your death streak by getting a kill, your value is reset to 300 gold. On the other hand, every kill that is done in a row without death increases the bounty to a maximum of 500 gold. The gold bounty is not reset when the champion is killed entirely by a turret or creep without any enemy champion contributing to it. Being on a killing spree does not grant any special bonuses. The first kill in a Match is referred to as First Blood, which is also announced. First Blood gives an additional 100 gold (400 total) beyond the basic amount for the kill.

Bounty values
Pre V1.0.0.120 values:

Multi Kill
If a player kills several heroes within a few seconds of each other, the following comments are shown: "Champion" has scored a [...]!'
 * Double kill (2)
 * Triple kill (3)
 * Quadra kill (4)
 * Penta kill (5)
 * Legendary kill (6+)
 * Legendary kills are extremely rare and can only be earned by killing the entire opposing team and killing enemies that have used.

There is no bonus associated with multi kills.

Ace
A player scores an ace if he kills the last living hero of the enemy team, leaving all opposing players dead. Note that scoring an ace is far easier than scoring a penta kill, since the only requisite is to kill the last enemy champion, not all of them. There is no bonus associated with aces.

Items benefiting from Kills

 * UNIQUE - Your champion gains +5 damage per stack, receiving 2 stacks for a kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion's movement speed is increased 15%
 * UNIQUE - Your champion gains 8 ability power per stack, receiving 2 stacks for a kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion's cooldowns are reduced by 15%.
 * UNIQUE - Your champion gains 32 health per stack, receiving 2 stacks for a kill and 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion takes 15% less damage.
 * Gain an additional 1 Physical Damage and 0.25% Life Steal per kill. Maximum of +40 Damage and +10% Lifesteal. Bonuses are lost upon death.
 * Permanently gain 4.5 Health and .15 Health Regen per 5 sec per minion kill. Champion kills grant 45 Health and 1.5 Health Regen per 5 sec. Bonuses cap at +450 Health, and +15 Health Regen per 5

Skills benefiting from Kills

 * (Active): Akali moves through the shadows to quickly strike her target and deal magic damage. Akali gains an Essence of Shadow once every few seconds up to a total of 3, affected by cooldown reduction. The time to gain a charge doesn't count when at maximum charges. Additionally, Akali gains an Essence of Shadow for a kill or an assist.
 * (Passive): Each time Evelynn scores a kill or assist she regains health.(Active): Evelynn gains attack speed and movement speed for 10 seconds (non-stacking, can cast while stealthed). Malice and Spite's cooldown refreshes every time an enemy champion dies.
 * (Innate): After a kill or an assist, Katarina gains 25 gold and her cooldowns are reduced by 15 seconds.
 * (Active): Increases Master Yi's movement speed by 40% and attack speed for a few seconds, as well as making him immune to any slow effects. Additionally, killing a champion refreshes all of Master Yi's Abilities. Assist refreshes all abilities by half.
 * (Active): Mordekaiser deals damage to the target and curses it for 10 seconds, stealing life. Damage is equal to a percentage of the target's maximum health. If the target dies while under the effect of the curse, their soul is enslaved and will follow Mordekaiser for 30 seconds. During this time, Mordekaiser gains 20% of their ability power and attack damage. The spirit gains 75% of Mordekaiser's adjusted damage, ability power, and health totals.
 * (Active): Tristana fires at the ground to propel herself to a target location, dealing magic damage and slowing surrounding units by 60% for 2.5 seconds when she lands. On kills or assists, Rocket Jump's cooldown resets.
 * (Passive): Veigar gains 1/2/3/4/5 bonus ability power for each champion kill from any damage source.