I'm reading into the question a bit, but my guess is that you're really looking to find out whether knights were harmful or helpful to society in general. As has been pointed out, individual knights' behavior could vary enormously, like every other group of human beings.

The short answer is that we don't really know: it's an area of huge debate among medieval historians. There's a school of thought, associated especially with Thomas Bisson, that the emergence of knights in Europe in the 10th century saw a huge wave of violence sweep over Europe, and that anyone who wasn't a knight's life got a lot worse, as knights went around demanding increased taxes and customs, and killing anyone who stood in the way. There's been a big backlash against this view in recent years, however, and many historians see knights as no more or less violent and harmful to society as military aristocrats in any other period. The question is pretty much still open though (and good thing too, as it's a major strand of my thesis!)

I'm on mobile right now, so cant write too much, but if you want more info on the debate and the evidence for each side, feel free to ask follow-ups, cause I could write for hours about this stuff.

You may need to clarify the question. What qualifications would make a specific medieval knight helpful or harmful? Knights were heterogenous individuals with wildly varying experiences, united under that term only by virtue of their status as prestigious and often wealthy combatants. Furthermore, what was helpful for one lord often was harmful for another in terms of loss of holdings, human capital, resources, etc. It's very nearly impossible to generalize without a much more targeted query.