Light Yagami/Harry Potter
First, Light should have just killed off all the Wammy's children in L's successor program back when he first came to know of them. Watari thought a 'Delete All' would remove all information stored in the database about L and his operations. A year down the line, a whole new shelf added to his bookshelf with books on Memory Management, Warehousing and Data-structures and some coding had him recovering 60% of the data.
Light didn't become the second L just for the Kira case. He actually had the data, some of the contacts and a steady trickle of international cases for all three of L's detective persona's. He knew of Wammy's long since. But see, here's the thing-
Light Yagami didn't kill children. He never went after children.
The boarding school, for it was a boarding school for orphans on records and not an actual orphanage, was attended only by children 20 and younger. Maybe young adults would be a better term but Light never went after anyone younger than 18. Maybe if he stretched his morals a bit, he would agree to go after an 17 year old.
And Mello, Near and Matt were children when he first came upon their records. He didn't make much of them. They had never met L, had no reason to avenge him, and their IQ scores were lower than him. Their social skills were only remarkable in their lack, even worse than L's.
Three years down the line, Light Yagami regretted not killing Mello, damn his moral code, when he had first seen that name.