I interpret this quote to mean that when passion takes control over us, bad things occur. Our passions, are our intense feelings or emotions: love, rage, hatred, etc. They are normal and can be un-harmful if we control them; "good servants". However, if we let them dominate, if they overpower, harmful or destructive consequences arise; " bad masters".
Love in its various forms is an amazing and wondrous thing. It's indeed worth living for, and its seeking is only natural. However, loving something too much can be a bad thing. People can get obsessed with one another to an unhealthy and self-destructive point. To a point where this "love" creates fanatics, creates stalkers. Not only is it a harmful thing for those with the affliction, but also to their victims. Terrible things occur when an admiration goes to far. Being in love is suppose to be great. But when that love is unrequited it is awful, it causes much sorrow and despair; as does the ending of a relationship. And it is not rare for that despair to lead into depression. Depressed individuals may harm themselves or even commit suicide. So, it is not untrue to say that people do quite literally die from a broken heart.
A love for a cause or belief can also got too far. Having morals, beliefs, spirituality, or patriotism are all admirable attributes. Nevertheless, the passion that one puts into those attributes can go too far. Many people associate themselves with a certain religion; (that being any thing from a large wide-spread faith to a small sect or cult). And out of those "many" there are some who let there beliefs overpower them. People in current day (as well as throughout history) have sacrificed themselves in the hope of being brought to the next life; hence mass suicide rituals. Others kill those whom they feel are blasphemies; the witchcraft persecutions of the sixteenth and seventeenth century. And also, in my opinion ones sense of patriotism or personal belief can also go too far. That going "too far" has created such individuals as suicide bombers, Kamikaze pilots, self-harming protesters, and those who find in "honourable" to kill themselves if their side loses a war. When the passion one feels towards their belief gets to point where they hurt themselves or another, they are no longer in control, their passion is.
Hatred is an unpleasant and binding feeling. It's awful. However, no one can possibly like everyone or thing making dislikement or hatred fairly normal. That is considering that that hatred is controlled. But what if you can't control that hatred? What if it controls you? A lot of hatred is unprovoked, undeserved, uncalled-for. A lot of hatred is prejudice. And what happens when this controlling prejudice-derived hatred occurs? Auswitz happens. Slavery happens. Segregation happens. Martin Luther King in a coffin happens. Gay bashing happens. Neo-Nazis happen. Again, hatred is awful.
People must be in control of themselves or terrible things can and do happen. We can't let our passions have power over-us. That is a mistake people make over and over again. We let ourselves become overpowered by love or likewise by hate. We become to passionate with our beliefs, we let them do the thinking for us. Our passions are complicated emotions which in the long run may be beneficial to us, however, they can and often do destroy us.