Moral codes are not universal truths, but instead rules of engagement for maintaining order within a system, and they exist within every social scope, though their level of detail tends to decay as the scope becomes more interpersonal. They're not really a tool of the state, but instead just a human tool. The state just codifies its own and disseminates it into the social collectives it rules.

Yes, moral codes are generally arbitrary. When wielded by the state to maintain the status quo, it becomes a tool of the state.

My statement above is a moral observation about political morality within the US, and which I view is generally a useful rule within any democratic political system (I am referring to systems which have a structure and voting system associated with democratic processes, not necessarily ideal or actual democracies).

The US isn't truly democratic. Both major parties serve the interests of their donors, ie huge Capitalists, and the candidates presented fit with that alignment. In this manner, political pressure outside the scope of "civility" is presented as immoral, despite civility itself being used to perpetuate anti-democratic structures.

I am also not saying that this moral code is necessarily good for us or the system itself at any given moment, but stating why this moral code exists in the first place, and why anyone who is apart of our system and wants that system to survive (whether that be for avoiding personal turmoil or political ideology) will continue to condemn assassination attempts from any side.

Yes, this is why Biden has batted more for Trump than any child murdered in Gaza. Biden needs civility to remain, or else he too will become a target.