The game only counts _kills_ when ranking players. Assists and deaths are not used, except implicitly in the kill score in the case of suicides.  

When two or more players are tied, they are sorted by the time of their final kill: an earlier final kill is ranked above a later one. This is also true in Team Deathmatch if the two teams are tied when the time runs out: the team who scored their final kill the earliest wins the match.