Unwholesome thoughts are those that (are not conductive to freedom and happiness, but rather) promote suffering.
Unwholesome thoughts (also called unskillful thoughts) are connected to selfish desire, hatred, or delusion.
Therefore, selfish desire, hatred, fear, envy, and (especially) delusion promote suffering.
There is a particular pattern of unskillful thinking ("unwholesome thoughts") that most of us regularly practice nearly instinctively. It is a manner of thought strongly encouraged by our competitive culture. (and also fosters zero-sum thinking).
We have the insidious habit of seeing how we measure up to other people ("comparing mind"). Our culture is obsessed with it and thrives on it.
The most damning effect of this 'comparing mind' is the unnecessary suffering it causes. Not only do we feel bad about ourselves, but we begin to wish ill upon others - sometimes to the point where we are willing to take actions to realize those wishes. This is the birth of Envy.
Not only is Envy a stumbling block and obstacle to Compassion, but Envy fetters Compassion.
Compassion is a form of empathy, but much more than that. Compassion is action. It will not just sympathize, it'll do something about it. To only sympathize is Pity. And Pity is a very pale reflection of Compassion.
What will block and impede Compassion is Envy. Yes, there is fear and jealousy, but Envy is much more dangerous and damaging. With Envy there is also the intent or desire - either directly or indirectly - to inflict misfortune or harm onto another. Not only do you desire what they have, you don't wnat them to have what you possess (because what you possess is a commodity and makes you powerful). It is Compassion's opposite and a stumbling block.
The capacity for compassion is our deepest nature as human beings. I believe we are all innately good.
(...and on a side note, I also think that doctrines like the Calvinists Total Depravity only further feeds Envy and slowly eliminates Compassion, allowing them to only - at best - show Pity).
Compassion does not require that we feel affection towards others. We can have compassion for our worst enemies. In fact, the very nature of true compassion might necessitate it. Compassion cannot be selective.
Compassion is not Pity, although the two are often easily confused. Pity is simply feeling sorry for someone who has to endure suffering, but pity keeps itself distance from suffering. Pity can't get past the element of fear; it's afraid of pain and suffering and wants to flee from its presence.
Compassion doesn't keep its distance. Compassion is willing to be with suffering up close and personal because it has learned to accept rather than resist pain.
Although Dharma is a Buddhist term, I don't see the concept as exclusively Buddhist. I believe the Buddhist's understand it and draw from it, but don't "own" it.
Dharma is our openness and willingness to accept the truth as we discover it. Our receptiveness to it and our ability to allow it lead us, rather than force it to fit our preconceived notions and agendas. Our willingness to allow it to change and direct ourselves rather than twist and coerce and manipulate it.
It is dealing with the struggle and tension between truth and fact. (as they are not the same things).
The truth never needs defending. This is Dharma. Only our truths needs defending. This is Delusion.
If we are open and aware and allow ourselves to be led where ever it is that the truth leads us, we consistently reduce the levels of delusion in our lives.
Delusion is Dharma's enemy.