It's a loose end. Two lines on the bridge at the end woulda done it. "Gosh, Spock, who woulda thunk his reputation was a sham?" Or, "Gosh, Spock, too bad such genius went mad." Or whatever. It just seems weird that his rep is so played up, but when we meet him, he is the "bad guy."
There's no farewell scene or line with Helen, either. Maybe the ep was running long.
The Revised Final Draft script of "Dagger of the Mind" (August 5, 1966) offers a bit of extra dialog that helps flesh out Dr. Adams' motivation a bit.
From Act Four, Scene 177--Kirk is in the Tantalus device chair for another treatment:
ADAMS
I give you credit. Van Gelder was
on his knees sobbing my now.
(nods)
It's good I've had a pair like you.
I've learned a great deal.
KIRK
(with difficulty)
For... what purpose, Doctor? I
cannot understand a man of your...
of your...
ADAMS
Of my reputation? Unfortuanately,
I have little else... except...
now, with this device... power.
Power over minds... and thus
over everything that counts.
The final great criterion.
Intriguing. And since I have
it, I've decided to use it for
myself... after all these years
of doing things for others.
(beat... a smile)
Say I want a very comfortable
old age... on my terms... and
I am a most selective man.
KIRK
Unnecessary. Just... trust...
ADAMS
TRUST mankind to offer me my
just rewards?
(smiles)
You're an optimist, Captain.
In this work I've learned too
much about men's minds.
(interrupted by
Lethe hurrying
into scene)
LETHE
The woman doctor... she is gone,
Doctor Adams...
At this, Adams throw a look at Kirk, reaches for the
panel, turns up the volume, the device HUMMING STRONGLY.
********
So, it appears Dr. Adams is an embittered, disgruntled employee, tired of constantly taking care of others. Dr. Adams simply went "postal."