It is an axiom of the law of evidence in criminal law, for centuries, that one's character cannot be used against one in a prosecution. The theory is that people should be convicted of crimes based on the evidence of these crimes, and not based on their character. Consider how easy it would be to convict just about anyone of a heinous crime if all one needed to present at trial was the fact that this person was a really bad person who had done bad things in the past.
For example, Charles Manson (generally the poster child for evil) is currently housed at Soledad Prison in California. Presumably, people get killed there with some degree of frequency. Assume that someone dies there, and it's unknown who did it, but perhaps Manson had the opportunity to do it. Imagine how easily a prosecutor could get a conviction against Manson if all he had to do was present evidence of what an evil guy Manson is? There would be no need for real evidence linking Manson to the crime, because most juries would be inclined to convict Manson no matter what.
However, California courts have, over the years, stripped away at that theory. Under Evidence Code Section 1101(b), and due to several California Supreme Court rulings, more and more prior bad acts of defendants are now admitted for purposes of proving that they committed the present offense.
A couple of years ago I had a murder case that was a "whodunnit." No issue at all as to motive, or intent, just who did it. There were serious doubts that the defendant was actually the one to commit the crime. The prosecution tried to present evidence that my client was convicted over a decade earlier of robbing a liquor store with a gun in order to prove his intent was robbery in this case. It was outrageous, and the judge fortunately forced the prosecution to accept a stipulation as to all elements of the crime except identification in lieu of letting in the evidence (the prosecutor would have much preferred to have brought in the priors, being far more harmful to my client than the admission that the crime was committed for purposes of robbery). Of course, the case hung, and was set for retrial. Another lawyer did the 2nd trial, and this time the judge let in the priors, and this time the defendant was convicted in mere minutes, and sentenced to life without parole plus an extra 200 years or so for good measure. the case was reversed (thank goodness, the person really was innocent).
Which brings me to Phil Spector. As noted in this LA Times article, the prosecution is bringing in evidence that Spector has menaced women with guns in the past, and harbors animus towards women in general, treating them with contempt and violence. The situation here is different, though, and actually points to areas in which the courts have probably correctly ruled that this type of evidence is admissible.
In the Spector case, the defense has alleged that the victim killed herself, or that the incident was an accident, or mistake, but that Spector never intended to kill the victim. The prosecution seeks to present these prior incidents to show that this was no mistake, or accident, but that this is consistent with Spector's pattern of activity. Consider the contrast with the case I had, where the only issue was "whodunnit," and the prosecution was putting on this evidence as straight character, for purposes of identification. Here, the evidence would rebut Spector's claim that what happened was not a volitional act on his part. This would be a more correct interpretation of the law (I'm convinced that the judge in my case was either really stupid, or really evil, or both - and he was reversed).
There is little danger in the Spector case that he will be falsely associated with the case due to this evidence - it was at his house, and she died with his gun, and he was the only one there. He has presented a scenario of how the crime took place that is at odds with his past. It's not like he's saying "I didn't do anything," and they seek to use evidence that he slapped his wife before to show that he's done "something" in the past. Here, the victim is killed in a relatively peculiar way, and the suspect denies involvement by trying to exculpate himself with a specific story. However, it's been shown that has acted in a very similar manner to this before.
It's a close call, but not that close. Judge Fidler has let some of this stuff in previously, I'm convinced that he'll let this in as well. If he excludes it, it will only be due to the lateness of the discovery, and probably not do to the relevance objection.