Some possibilities that I have encountered.
Possibility #1
Friendly fire is active (I believe this is the default setting). In many games, battle often turns into a furball and many teammates suffer damage or death from friendly fire. This can't be avoided. Almost all the servers I go on have Friendly Fire turned OFF or have friendly damage reduced to 25% or something along those lines. Playing on these servers (which are the majority) is much more fun.
Possibility #2
Your team hates you for some reason. Either they don't like you, or they are just an entire team of traitors. Switch to the other side and play or leave the server.
Possibility #3
Your skin is not showing. Often customized skins don't work and you are displayed as a neutral green color. No one can tell which side you are on, so everyone attacks you. Change your skin back to a default and play.
Possibility #4
You have a single team traitor who is following you around and killing you all the time. Because he is an exceptionally good player, it may seem like your entire team is killing you. Hit the "`" key the next time you get killed and look for your name under who killed you. Explain the situation to everyone else there (repeating several times if necessary. If nothing is done about the problem, switch teams or leave the server.
Possibility #5
Cheats. I have encountered players who cheat. They are extremely rare, but it happens. I have encountered players that were totally invisible. And I mean totally. Ping is low for myself and everyone else. I see no shimmer or telltale sign at all. I see nothing but shots coming out of nowhere. At times, all you could see was their weapon when it fired. Other cheats I have encountered are flag carriers. They look like they have the flag because their skin picture includes it. Hence they go around looking like the flag carrier when they are not. Solution is to leave the server, report the cheater on message boards and the server number.
Possibility #6
You haven't got the latest version of Unreal Tournament. Sometimes problems occur with cross version games. You are using one version of the game, while the server is using another. There have been several patches realeased and a bonus pack for Unreal Tournament. I suggest downloading and installing the latest patch and the bonus pack.
Possibility #7
Your ping is so bad that, although you are moving around and shooting, you are actually several seconds behind everyone else. This is casuing you to suffer greatly. You are either skipping around from everyone's perspective or getting in the way and being killed, whether by friendly fire or opposing fire. Reconnect to the server, reboot the game, or reboot your computer and try again.
I hope this has helped a bit.