Can I customize "killed" and "killed by" messages in MP games?

Yes you can, says OFP player [P-A-S]Paynemaker.

The templates for these messages are in file STRINGTABLE.CSV. For OFP: Resistance, the file is located in subdirectory \RES\BIN. For older OFP versions, the file is located in subdirectory \BIN.

You can edit these files with a simple text editor but make a backup first before you do. Search for the string "STR_KILLED". There are four matching statements. Each statement contains a message's template in English, French, Italian, Spanish, German and Czech. Modify the text in the templates as you desire but do not remove or change message variables, such as "%s"  and "%d".