This evil puppy manipulated the three since childhood.
Kelley was a member of the Howling Voice Guild who, growing up in the same "garden", quickly formed a bond with Elza and Clive. The three of them would aspire to become Knight class gunners, eventually becoming the first group in several years to pass the tests.
The elders of the Guild became suspicious of the three, especially as they were apprentices of Sauro, one of the other elder, fearing an imbalance in power within the guild. Finding Kelley the easiest to manipulated, they appointed him as the new Guildmaster and so Kelley slowly turned his back on his childhood friends.
It's very sweet but they're already highly trained killers.
One day, Elza was asked to duel Kelley, which was most probably a scheme pulled by some of the elders: Kelley was not able to hold Storm, the weapon of the Guildmaster which is said to have its own will and to chose its own master, but Elza could handle the weapon with no problem.
In order to secure Kelley's position, the elders ensured that the gun Mond would be loaded with blanks, allowing Kelly to kill Elza easily. However, Kelley intentionally chose Mond, representing betrayal, leaving Elza to choose Stern, representing trust in the Guild, and Kelley was killed in the duel. Elza, fearful of retribution, escaped easily as Kelley had ordered all gates to be left open, in order to secure his friend's escape.