Being Good Hearted Necessary to Practice?

      I was reading a translated material from a Buddhist text today, which stated that being "good natured" is a basic prerequisite to following Buddhism. After all, a person can spend a lifetime reading and diligently studying scriptures, yet, may not fully benefit from such a practice if they are not kind-hearted to begin with. Some Buddhist teachers even recommend that people stay away from Buddhism altogether if they lack the basic disposition of kindness. This, I assume, is to ensure that Buddhism's reputation as a belief system is not harmed by people who lack that required goodness.
   I think that such kind of a recommendation is a very good reminder that people should approach Buddhism with the intent of uplifting others and oneself. However, it leads me to wonder: if those who start to practice Buddhism lack a disposition to be happy or good-natured, would they not benefit even more from being a Buddhist? I am thinking in particular of famous "Buddhists" in history who killed, such as Milarepa, before being suddenly changed by a particular teaching or even a turn of phrase. Some of the most dramatic conversion narratives in religion, in fact, arise when people who are "mean" spirited are suddenly turned in the opposite direction, if not simply by the grace of a higher being. Paul in the Bible is one such example of a person who starts out as persecuting Christianity, only to later make a 360 degree turn.
  If people are only allowed to practice a religion when they have a specific temperament that is benevolent, a lot of others end up being excluded from it, including those who do make sudden progress after a period of drudgery and study. Perhaps what was meant by this translation is that one should, regardless of temperament, always try to look to the best parts of themselves when taking on a spiritual practice. Otherwise, it can be very easy to slip into looking at spirituality through a defensive or uncharitable lens. I also assume that even if we might not have a natural disposition to be friendly, at the very least, we can cultivate humility to this fact and start to work on softening our character and lightening up a bit. After all, it's through a certain kind of softness and openness that these teachings can slip into the cracks of the heart.