The Homunculi names in Full Metal Alchemist 2003
As far as I know, there is no certain canon answer but there are quite some hints.


At the end of FMA03 we see Dante doing something to Gluttony removing his intelligence and personality and replacing them with an endless hunger for energy. So Dante being directly responsible for the 7 sins symbolism is not out of the question but it does not quite add up.
Since every homunculus follows the theme of their name, we can rule out the possibility that it is just Dante calling them names.
This "spirits of homunculi"-idea is based on Wrath (Sensei's homunculus), right? He tells Ed about his time behind the gate and how he acquired Ed's limbs. But he never said anything about originating there. Unlike Manga or Brotherhood, there is no connection between Homunculi and the shadows behind the Gate. Sensei created Wrath after losing her child and after realizing her mistake, she put him on the other side of the (still open) Door. 2003 Humunculi are just sentient bodies that are very similar to humans but have neither soul nor a healthy personality.


I originally thought that the 2003 homunculi followed the sin that led to their creation but while Lust was created by her lover, Envy was created by Hohenheim back when he thought himself to be omnipotent. So it is safe to say that the homunculi are by nature always "born" into challenging circumstances and are unable to form meaningful relationships to process them. Dante might influence them to make them easier to control but even without her, they are unhealthy beings that are driven towards spoiled and extreme personalities.