I think is a natural attitude to hate a guy who played for your team and suddenly moved to a rival. You feel betrayed and you don't give a damn to the same old story "well, he is a professional", in that precise moment that boy is your #1 enemy.
But, in a certain way, Gerd is right. It might be related with cultural aspect, at least when you saw Marca's list on the 20 biggest traitors in football history:
1. Figo (Barça -> Real Madrid)
2. Roberto Baggio (Fiorentina -> Juve)
3. Ruggeri (Boca -> River)
4. Sol Campbell (Tottenham -> Arsenal)
5. Batistuta (River -> Boca)
6. Romario (Flamengo -> Fluminense)
7. Cruyff (Ajax -> Feyenoord)
8. Caniggia (River -> Boca)
9. Hugo Sánchez (Atlético Madrid -> Real Madrid)
10. Mo Johnston (Celtic -> Rangers)
11. Tardelli (Juve -> Inter)
12. Luis Enrique (Real Madrid -> Barça)
13. Krancjar (Dinamo Zagreb -> Hajduk Split)
14. Ince (West Ham -> Man Utd)
15. Laudrup (Barça -> Madrid)
16. Aldo Serena (Torino -> Juve)
17. Cáceres (River -> Boca)
18. Denis Law (Man Utd -> Man City)
19. Schuster (Barça -> Real Madrid)
20. Gatti (River -> Boca)
Conclusion: 90% of the cases are from Latin and British cultures.
I'm sorry, it's a bit (or totally) off-topic talk but very interesting in my opinion.