What's a parallel for 'mitigate', for worsening a good situation?

Solution 1:

If you are looking for a word that means to make a good thing less good, rather than to make a bad thing worse, a possibility would be vitiate:-

To reduce the value or impair the quality of. [American Heritage Dictionary via The Free Dictionary]

to make faulty or imperfect [Collins English Dictionary via The Free Dictionary]

Solution 2:

What of undermine?

If a good thing is proposed or implemented and something else weakens it, I would think that undermine would be a good choice:

Merriam Webster has these definitions (among others):

to subvert or weaken insidiously or secretly

to weaken or ruin by degrees

Solution 3:

To mitigate means to lessen the severity of a situation. You're looking for a word to make a situation worse. There are a couple of multiple antonyms at the same level of formality:

exacerbate
aggravate

More informally would be

worsen

Solution 4:

Compromise : To reduce the quality, value, or degree of something; damage, put in danger.

E.g:

The affair seriously compromised the party's prospects of success

Solution 5:

I'll go ahead and submit that the author already used mitigate properly. My printed copy of the American Heritage Dictionary, Second College Edition gives this, and only this, definition (and etymology):

To make or become less severe or intense; moderate. [ME mitigaten < Lat. mitigare < mitis, soft.]

Several people have focused on the "less severe" part of the definition, which is fair. I don't think anyone will dispute that mitigate is most commonly used to mean "to make something bad less bad". But the definition above also says "or intense", and surely good things can be made less intense.

Regardless, I would argue that "to make milder" much more accurately captures mitigate than "to make better", or even "to make less bad". As such, I feel the author's intent was clearly, and perhaps even a bit colorfully, expressed.