Evil thoughts can destroy a person's well being
Hindu Shastras has identified primarily six inner foes of mankind. They are collectively called the Arishadvargas or Shadaripus.
They are kAma (lust), krodha (anger), lobha (greed), moha (delusion), mada (arrogance) and mAtsarya (jealousy).
All the evil thoughts that arise in our minds are triggered by one or more of the above 6 foes.
For example from lust arises illicit thoughts about sexual gratification, from anger arises the evil thoughts of destruction and revenge, from greed arises the desire of owning things illegally etc.
In Vedas there is a mantra that symbolically identifies these 6 foes as six relevant animals. It also asks Lord Indra (the king of Gods) to kill them all for us.
UlukayAtum shushulukayAtum jahi shvayAtum uta kokayAtum suparnayAtum uta grdhrayAtum drshadeva pra mrina raksha indra.
O Indra, kill the delusion (identified as the owl), anger (wolf), jealousy (dog), the lust (chakravAka, a kind of bird), arrogance (eagle) and the greed (vulture).
Atharva Veda Samhita 8.4.22
In Sanskrit literature the chakrvAka bird is often described as being stricken with love and lust. So, its being associated in the mantra with lust or kama.
For dog we have a popular saying which says
" dog neither eats the grass nor allows the cow to eat the grass in the manger."
The dog is associated with jealousy or matsarya in the mantra.
Vulture can mean " a person of rapacious and predatory nature." Vulture is associated with arrogance and so on.
Basically, any thoughts that can be traced back to anyone of these aforementioned six foes of mind is detrimental to our spiritual progress and hence can be marked as "evil".
So, we have to monitor them and try to put a check on them. But obviously that is easier said than done.