person who's too eager to ascribe malicious intent

Solution 1:

Cynical,

believing that people are motivated purely by self-interest; distrustful of human sincerity or integrity.

Apprehensive,

anxious or fearful that something bad or unpleasant will happen.

The person may actually just be narrow-minded,

not willing to listen to or tolerate other people's views; prejudiced.

Or biased, discriminatory, etc the list goes on for intentional suspicion.

I think such people are prone to projecting themselves (experience, history or childhood) unto others and hence, their thinking is always similar (in this case, ascribing people's motive as bad).

Solution 2:

Idiomatically, one word that could be used in such contexts is...

"An uncharitable person might think [that malicious intent was a factor]"

Solution 3:

That person could be described in a semi-slang way as paranoid if the evil plotting and malicious activity is directed at them.

Solution 4:

The word I would use is suspicious.

Solution 5:

A person who is abnormally suspicious is querulent

Denoting one who is ever suspicious, always opposing any suggestion, complaining of ill treatment and of being slighted or misunderstood, easily enraged, and dissatisfied; characteristic of paranoid personalities.