Alien Swarm (in Steam) is a top-down shooter game and there are a hell of a lot of mods for it via developer console, to enable this navigate to the Options -> Keyboard/Mouse menu and check the option labeled "Enable developer console". The keybind to open the console can be changed in this same menu. By default it is bound to tilde (~).

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To go Rambo (on your own) type this in the DC

asw_ignore_need_two_player_requirement 1

To add Bots

alias “addbot_Sarge” “cl_selectm 0 -1″
alias “addbot_Wildcat” “cl_selectm 1 -1″
alias “addbot_Faith” “cl_selectm 2 -1″
alias “addbot_Crash” “cl_selectm 3 -1″
alias “addbot_Jaeger” “cl_selectm 4 -1″
alias “addbot_Wolfe” “cl_selectm 5 -1″
alias “addbot_Bastille” “cl_selectm 6 -1″
alias “addbot_Vegas” “cl_selectm 7 -1″

you type in just cl_selectm 0 -1 for Sarge

Have fun :)