I don't believe that everyone who gets cancer deserves it and that it is some sort of karmic payback for their prior bad behavior, but if hold you that superstition, ok...
A criminal trial is not about seeking justice for the victim, it is about the prosecution meeting the burden of presenting proof to the jury that the defendant committed the offense beyond a reasonable doubt. In this case they failed to do that. From Alan Dershowitz:
http://www.huffingtonpost.com/alan-dershowitz/the-criminal-trial-is-not_b_893207.html
The Criminal Trial Is Not About Justice for the Victim
A criminal trial is never about seeking justice for the victim. If it were, there could be only one verdict: guilty. That’s because only one person is on trial in a criminal case, and if that one person is acquitted, then by definition there can be no justice for the victim in that trial.
A criminal trial is neither a “whodunit” nor a “multiple choice test”. It is not even a criminal “investigation” to determine who among various possible suspects might be responsible for a terrible tragedy. In a murder trial, the state, with all of its power, accuses an individual of being the perpetrator of a dastardly act against a victim. The state must prove that accusation by admissible evidence and beyond a reasonable doubt. Even if it is “likely” or “probable” that this defendant committed the murder, he must be acquitted, because neither “likely” nor “probable” satisfies the daunting standard of proof beyond a reasonable doubt. Accordingly, a legally proper result — acquittal in such a case — may not be the same as a morally just result. In such a case, justice has not been done to the victim, but the law has prevailed.