What if...? - My last minute theory

[vmonte]

No: HPFGUIDX 172073

I wrote:
No, I meant that he (Dumbledore) may have had a checkered past.
He may have not been a good guy when he was younger.

Miss Vassy responded: 
Sorry that I missed the point. I agree that Dumbledore probably has a 
checkered past. I wonder if it has to do with the Grindewald?

vmonte again: 
Yes, I think so. JKR would have given us Grindewald's life story 
before now, unless there was something unpleasant about Dumbledore she 
didn't want anyone to know. Did they start out as friends? I'll 
probably be wrong about this too. lol