A lot of the Delta STs are Type 1 (in the Enneagram), which is exactly about inner rage and the suppression of such, through the ideal and pursuit of perfection.
Seems like most Delta STs you've known were Type 1.
I can say that most SLIs I met were most likely Type 1.
The healthier ones are much more balanced and flexible. The less healthy ones are very strict, tight-up, and difficult to deal with (esp. as an IEI).
Then there are also Type 9 SLIs. Type 9 being a fellow Gut Type, they also have Anger issues which are even more repressed than is the case with Type 1.
(That is why Type 9s are usually rather easy to get along with, whatever their Socionics type – their anger is so repressed, it hardly ever comes out.
But boy, if it does... It can resemble a Type 8's anger, esp. when they are a 9w8... It can be scary.)
At last, the third most common Enneagram type for SLIs is Type 6. Those are the more cerebral SLIs...
All in all, I attribute those anger issues (besides Enneagram type) to weak . Because IME, all types with weakened had anger issues, and general issues with expressing their emotions in a sensible manner.
Interestingly, unhealthy leading types will act very similarly to types with weak .