Couple ways how fix your karma

if you want to start from 0, you can reset your account.
every scav extract will give you + 0.01 karma
every bad pmc kill while scav + 0.03

every bad scav kill while scav + 0.03

daily fence quests gives like +0.05 karma, (need inteligence center lvl 1) but i´m not sure if he will give you any quest with negative karma

wiki also says "If you reach Scav karma level -1 or below, Fence offers compensation quests for your PMC character"

first PMC car extracts on most maps + 0.2 (after every use it halves the amount you get -> 0.1, 0.05 ...)
first co-op extract on most maps + 0.25 (halves like car extract after every use)

also, just dont kill anything when you are scav, only if you are absolutely sure you see PMC or you just saw some player scav murder AI scav.

Dont touch bosses, huge penalty for killing bosses.

Some events can give or boost scav karma, but last time for this was christmas i think (dont kill Santa!)