Many states model their evidence rules after the Federal Rules of Evidence, including Colorado. Rule 403 states: Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.
The reason this rule exists, besides being based off the California Code, is because the jury should come to a verdict base on the facts and not a visceral emotional reaction. In an instance like this the jury may believe he really was insane, but not care. At that point they completely invalidated the intentional component of intentional murder.
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