Is there a console command or something? I want to be able to do one-hit kills with the pistol in single-player mode

yes, there is a console command, but you may need to re-enter it each map change.

something like this:

sk_plr_pistol_damage or something, it will pop up as you type “sk_plr_” giving you a list of weapons. (actually, it could be plr_sk_)

but just find a pistol, press Enter once on it to see what it currently is (eg 8 or something) then change it to like 90, and change it back when you’re done.

upping damage may cause physics to go a bit haywire, so you might want to change phys_pushscale to something a little under 1 so that bodies react less to your superpower pistol shots (eg, phys_pushscale 0.6) but that’s not essential… just muck around and see what you like.

Look for a mod, if you know-how, look for a text file in the GCFs and edit that, can’t be too easy though.

This is the exact command, from skill.cfg:
sk_plr_dmg_pistol “5”

You could either do this by typing that command in the console, like blink said or if you don’t want to type it in with each map load, you can extract Half-Life 2’s skill.cfg from source engine.gcf/hl2/cfg and place it in the your_username/half-life 2/hl2/cfg folder and change that command in it.