While the two can be taken in the same way, the difference in wording generally leads to subtly different meanings.
His former friend wouldn't have left him to die. It is just too heartless.
This says that the act of leaving somebody to die is heartless. It refers to the act in general and would apply to anybody leaving anyone to die.
Because it is heartless, it's something that is true regardless of whether or not his former friend actually did it or not.
His former friend wouldn't have left him to die. It would be just too heartless.
This can have a more specific meaning, referring to it being heartless for his former friend to leave him to die.
In other words, it applies to the specific set of circumstances (his former friend, whoever he is, and whatever the situation is), rather than to leaving people to die in general.
Since it's conditional, the statement is only true if his former friend did it.
If you want to make a statement about leaving people to die in general, then use is. But if you are making a statement about the particular situation (which may or may not have happened), then use would be.