It's easy to put this down to Field being an exceptional psychopath, which he absolutely is. Absolute evil in every way, his poor parents as well as his victims.
But scamming the elderly is quite common. I watched a few episodes of Love or Money on iPlayer and in all cases it was lonely older often widowed people who were flattered a better-looking younger person came along offering companionship and love, and didn't ask any questions about why. It's tragic to watch as it's extremely obvious from the outside that these are not genuine relationships and the 'love' poured out in texts/emails is simply not real and just a ploy for money. Some in that programme have lost over £100,000 plus. They did seem remarkably accepting though that these 'out of their league' people suddenly fell for them, and it's interesting to think why as most of us might be highly sceptical, we think, at least.
In Field's case he is doubly evil because he lived with and pretended to love these people in real-life which takes a particular type of enjoyment in manipulation. But it's not unique that people cut off from the fact the 'mark' (victim) is a real person with feelings and just see them as a source of money.
I was scammed online and on the phone (in a double targetted approach) and that was the worst thing for me, that the person on the phone pretending to help me knew they were scamming me, as I panicked and cried (losing a huge amount of money) and they simply didn't care about me as a human at all. I did get the money all back but that feeling that some people are just bad humans is a horrible one, even a tiny taste of it.
The acting in this is superb, Timothy Spall is so believable and pulled me in... so terribly sad.