Hi! I’m currently working on something similar. First of all, I didn’t get to the implementation phase yet, but here’s what I have planned:
As you know, the Game mode only exists on server, and kills and deaths should be tracked by the server. So what I thought of is having a function/event on the gamemode, and have a server event on the players that gets called whenever a player/bot dies.
This ensures that the actual management of those values can only be calculated on the server.
On that event, you make the necessary operations, and you can even “build” the data for the UMG killfeed. Then, in the same server event, you can call a multicast event that sends that UMG and the necessary data (as KDA counters or points or whatever) to the clients.
Again, I didn’t get to implement it yet, so I don’t really know if it can work, but in theory it should.
Anyways, I hope it can help you, and I’ll try to update you on it once I get to do some coding 
Good luck!