How could Wigglesworth's work have been both comforting and frightening?

The poet-minister Michael Wigglesworth became well known for his Puritan poem titled "The Day of Doom," which depicts Judgement Day as God sentences all men to Hell. Part of what makes Wigglesworth's poetry so damning is his complete refusal to show mercy to anyone. The poet spends line after line listing many different kinds of people whom he would expect to see in Hell. One particular passage reads:

Blasphemers lewd, and Swearers shrewd,
Scoffers at Purity,
That hated God, contemn'd his Rod,
and lov'd Security:
Sabbath-polluters, Saints persecuters,
Presumptous men and Proud,
Who never lov'd those that reprov'd;
all stand amongst this Crowd

Basically, Wigglesworth doesn't leave much room for anyone to escape eternal damnation. Of course, these ideas would be terrifying to many, particularly those who would prefer to believe in a merciful and forgiving God.
Much of his work focused on taking a similar path, with poems such as "Vanity of Vanities" and "To A Christian Reader" taking morbid and unsentimental approaches to physical life. The prior poem, for example, seems to suggest vanity to be a sin, particularly because he believes that Death is far more powerful than beauty. Basically, his poetry is very much of the proverbial fire and brimstone that makes much religious imagery graphic and frightening.
As to how his poetry is comforting, it would be plausible that like-minded Puritans would agree with Wiggleworth's damnatino of sinners, taking happiness and comfort that their way of life, the right way of life (they believe), will possibly spare them in the domain of the Lord.
https://biography.yourdictionary.com/michael-wigglesworth