This is because an attempt to commit any crime requires that the person specifically intend to commit that crime. You cannot be convicted of attempted robbery if you did not intend to rob.
Murder, on the other hand, may be committed when the defendant kills someone with an intent to kill. Or it may be committed when the defendant did something extremely dangerous with the conscious disregard for human life. This is called implied malice.
An example of implied malice could be where someone randomly shoots a gun in a place where people might be, if the shooter did not intend to kill anyone. Another example could be driving at a very high rate of speed on a road where the driver knows other people or cars may to be.
It is important to note that in most states, for murder to be of the first degree, there must be an intent to kill. A conscious disregard for human life will only support a lesser degree of murder, such as second degree murder in California.
Because murder can be supported by a lesser mental state than attempted murder, this could lead to the strange scenario where someone could be convicted of murder if the victim dies, but could not be convicted of attempted murder if the victim lives.
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