In Myanmar for the past several months revolutionary forces have been regularly threatening, targeting, and assassinating civilians who are believed to be reporting anti-government activity to the government (the civilian reporting results in arrest and/or death of revolutionaries and/or sympathizers). Would the assassinations be considered a violation of Geneva Convention and/or other international laws due to targeting non-combatant civilians, or are assassinations in that context considered a normal/expected part of a civil war? What factors go into determining whether or not killing targeted non-combatants is considered a war crime?
What factors go into determining whether or not killing targeted non-combatants is considered a war crime? The cynical answer is "it depends on who wins and who judges".
There are various definitions of what constitutes a civilian, all of which are "negative definitions" (i.e. Not a member of an armed force(e.g. an army, navy, etc; actually wielding arms is not required) or "involved in hostilities", meaning not a partisan, insurgent, or other form of "belligerent"). Civilians should not be targeted by military actions under international law.
On the other hand, spies, especially those who's activities cause death, are explicitly not soldiers entitled to Prisoner-of-War status, and can be executed. There are some protections provided for spies, but those generally seem to be geared towards soldiers captured during "non spy" activities with regards to past actions as spies, as opposed to "in the field" spies...and it is unclear to me what that means for an "informant" who presumably gathers intelligence in their day-to-day activities.
To what extent an informant is a "spy" is unclear. Espionage consists of the access, generally on behalf of a state (or belligerent party), to information that is held by another and considered as confidential or strategic, in the military, security, or economic field. Identities of personnel is very much confidential and strategic information, so passing that information to the government seems to qualify as espionage. Whether or not this equates to being "involved in hostilities" is unclear.
The case could certainly be made that an informant, especially one who does not go out of their way to gather intelligence or receive compensation is a civilian. Likewise, the case could certainly be made that they are a de facto belligerent, being no different in their effective activities than a formal intelligence operative or artillery coordinates finder. Wars are messy; civil wars especially so.
In addition to the answer by sharur, it must be said that it is not so clear when and if the Geneva Convention applies to civil wars. It is mostly an international treaty and whether it also applies to inner state conflicts is widely debated and depends on the nature of the opposing groups and whether they can - in some way or other - be considered a "military force". See https://en.wikipedia.org/wiki/Geneva_Conventions#Application for further information.
Violence against civilians by the state or violence from civilians against their own state's representatives (whether they're legally in that position or not) is likely not covered by the Geneva conventions.