Each country views the degrees of murder differently, indeed, even within a country like the USA each country views the same with various definitions. How is it in your country or State?
First Degree Murder is the most serious. Most often, first degree murder is categorized as "deliberate" - that is, the defendant made a clear headed decision to kill the victim - and "premeditated" - the defendant actually thought about the killing before it occurred (the period for this can be very brief).
Ref. https://criminal-law.freeadvice.com/violent...rees.murder.htm
You can also be charged with first degree murder if you intend to kill someone, and in the process you accidently kill another. Transfer of intent can occur even when the person you intended to kill is unharmed. Not only does this apply to murder, but assault, battery, and theft.