To understand why the number 13 is considered to be evil, we must first consider the number 12 for a moment. A day is made up of two times twelve hours, a year of twelve months and there are twelve signs of the zodiac, and so on.
12 is therefore the number that forms a whole, a complete set. But 13 is 12 + 1.
The 1 that is added does not belong to the set formed by the number 12; it is considered a foreign element, and if it is not pure and disinterested, if it does not vibrate in unison with the whole to which it is added, the whole set itself is threatened. This is why the number 13 is seen as a difficult number that brings hardship and even death: the thirteenth card in the Tarot deck is that of death.
Now, we could also say that the 1 added to the 12 represents the beginning of another cycle or of another set.
In Initiatic Science, death is never seen as an ending or an end beyond which there is nothing more, but instead as the beginning of a new life.