He probably was jealous in a number of different respects. He resented his entire family to many respects which is why there was so much friction. He wanted to live off his family rather than to have to work hard in life. While a number of things were in the mix pure greed is the main motive at the end of the day.
The estate would allow him to live the good life and that is what he did until he was arrested. If he had not been arrested I am willing to wager he would have blown the entire estate living large until he was broke and then resorted to more crimes because he certainly would not want to go back to working at that point.
Criminals who invest wisely so they can live a comfortable life is what you find in movies and on television. In real life when they take down a big score they live lavishly which ends in 1 of 2 ways: 1) it alerts the authorities and they get caught or 2) they blow it all.
His dislike of the family certainly helped him to not regret the killings and enabled him to do it but the motivation itself was that money. The estate was valued at over 450,000 pounds which in 1985 was a hefty sum. Today it is still a nice sum but not one that would be seen as enabling you to retire. In 1985 though it would have been enough to retire on and not need to work ever again if you spent it wisely.
