As for your first question, it is extremely unlikely that there is a record of the "first" murder using a firearm. A historian of the 14th or 15th Centuries might have an account of somebody being murdered using a gun, but I doubt it would be noted as "the first". As for your second, again the specifics of that would be better left to a historian of a given era. However, it is unlikely that you'll find much civilian combat with firearms until the 15th century or later, when the wheelock was invented, which allowed for the carrying of a loaded, ready to fire gun for the first time. Prior to that, guns were fired with a slow burning match attached to the lockwork, and were impractical to carry ready to shoot except when combat was imminent.