A passionate misuse of magic is more tolerable than a deliberate misuse of magic?
Not exactly. If what Harry say and my understanding of it is correct. intent i.e: Passionate or deliberate, is only a part of the equation.
Magic, at least the kind Harry is using, works on the principal a kin to the 3 fold law we know in many witch novels.
Whatever you send out, eventually comes back to you. If you send out good things, you get good things back. If you send out vile things, you get vile things in return. However, it is a bit more complex. The intent of the wizard only play a part.
The final backlash a wizard receives depends on how much is reflected back to the wizard. In witchy novels, it is often said to be 3 times what you send out, but in the Dresdenverse, the backlash a wizard receives depends on the consequences of the magic not on the intent of the caster.
Example: a wizard is attempting to use magic to heal a person. The wizard has about 97% good intent and only 3% dark intent. If the result of the magic is good, and the person is healed right and proper, the wizard will receive the backlash from the 97% good karma. If the person is not healed properly and get crippled, the wizard will be penalize by the 3% negative karma. If the person is killed due to the wizard's magical healing attempt, the wizard won't just get the 3% negative karma, he'll get 3% negative karma multiplied by the black magic, let say 10 times, so the wizard will get 30% negative karma even though he has 97% good intent when he casted the magic originally. This is probably what happens to Molly, though in her case, I estimate the intent is about 80% good and 20% dark.
Now, let's say that the wizard is only pretending to heal the person. He actually trying to kill the person instead. In this case, the wizard has 3% good intent and 97% bad intent. Assuming the wizard did kill the target, he'll get 970% negative karma. He'll go insane fast this way, but even if the magic fail to kill, the wizard still get the 97% negative karma from the original intent.
In theory, if the wizard can have 100% good intent, even if the target get killed due to the magic, the wizard won't be tainted at all. But it is impossible. Not to mention that it is impossible for a mortal to have 100% pure intent, if the magic is truly casted using 100% pure benevelont intent, the magic won't be able to harm anyone in the firstplace.