“When you see the wolf hiding among the sheep, all the sheep become wild beasts”
I have experienced the same thing as you, being used as a tool as a result of being kind. People seek to benefit themselves easily without giving much thought to the means or the people who get hurt in the process.
“Once a wolf bites you, you fear it will happen again”
That David guy sank his fangs into you and left you with a scar. The same thing happened to me with someone who said she wanted to be my friend but was just a leech. That's why I'm very careful with the people I meet, because somewhere out there, there's a hungry wolf.