Alien Swarm is still a fun game, and after a quick session this evening I felt like giving a solo run a try. Problem is, if you go to “Play online” and try to start by yourself the game complains that “This mission requires at least two players playing”. Fortunately, the developers were kind enough to include an option to ignore this requirement. Open up the console (first you need to enable it under Options -> Keyboard/mouse), and enter:
asw_ignore_need_two_player_requirement 1

You may also want to enter the following to prevent people joining.

make_game_friends_only

For the optional bots part, you can add bots by using the following:
cl_selectm [character] [slot]

Quoting from wiki4games:

[character] is the character as a number from 0 to 7:
0: Sarge
1: Wildcat
2: Faith
3: Crash
4: Jaeger
5: Wolfe
6: Bastille
7: Vegas
[slot] is the slot number (from 0 to 3, or -1 to automatically pick the first available lot)
To remove a bot, use cl_dselectm [character] where [character] is a number from 0 to 7 as above.