Vayeishev, Breishis 37:2. Yosef’s Friendship with the Children of Bilha and Zilpa: No Good Deed Goes Unpunished.

Rashi says that the other brothers did not respect the children of the shfachos, and so Yosef was careful to spend time with them and show them his respect. See also Rashi in Breishis 50:16-when the brothers sent a message to Yosef, claiming that Yakov had left a tzava’a instructing Yosef to not take revenge, they sent the message with the children of Bilha, because Yosef was close to them-“ragil etzlam”.

So where were they when the others planned to kill, and then sold, Yosef? Why didn’t they defend him? Of course, the bnei hashfachas had the same complaint against him when he brought what they considered to be loshon hara to their father, but it is clear that there was a failure of sympathy happening here.

Rashi in Breishis 49:5, d’h Shimon v’Levi, says that Yakov, when he was giving the brachos, said that it was Shimon and Levi who were the primary instigators of the plot against Yosef.  Rashi demonstrates that this is the case because Reuven and Yehuda wanted to save him; the bnei hashfachos “lo haysa sin'asan shleima” because Yosef had befriended them; Yisachar and Zevulun would never have taken a stand in the presence of the older brothers; so it must have been Shimon and Levi. But even though their ‘sin’ah’ was not ‘shleimah,’ they participated enough to be considered equally guilty.

But simple observation shows that the pariah, when given a chance to join the dominant group by attacking someone even more disfavored, will join the attack with the greatest zeal. Along the same lines, the Lutzker Rov in Aznayim Latorah in parshas Mishpatim talks about a freed slave being the worst taskmaster. The psychological tactic in play is that the former slave is expressing his anger and relieving his memory of powerlessness and frustration by causing others to suffer. In order to validate his treachery, he will recast his erstwhile benefactor’s solicitude as patronizing.

I have heard that Rav Pam used to keep a stone on his desk. A talmid asked why it was there, and he said that there are some people for whom you do a chesed, and with the passage of time, they turn around and want to stone you; he kept the stone on his desk in the hope that at least they should use a small stone instead of a big one.

I would suggest that a solution to this problem that you should communicate your feeling that it is your honor and privilege to be able to help them; or, use the Rambam's (10 Matnos Ani'im 9, הנותן למי יתן ולא ידע העני ממי לקח. כגון גדולי החכמים שהיו הולכין בסתר ומשליכין המעות בפתחי העניים. וכזה ראוי לעשות ומעלה טובה היא) "high level of Charity" simply out of a need for self preservation: ensure that the beneficiary of your chasadim does not know who you are.