Rav Yitzchak Grossman
Answer: This is a major machlokes haposkim.
There actually are conflicting Gemaras that discuss whether it's appropriate to give away one's estate to tzedakah. In one Gemara, someone is criticized for doing that. In another Gemara, it says that Mar Ukva left a lot of his money to tzedakah, and praises him for this. There, the Gemara adds that although a living person may not give more than one-fifth of his money to charity, at the time of death one may give more than one-fifth.
The Rema rules that there is no limit to how much one can give to tzedakah at the time of his death. Other Poskim rule that one can give up to half of his estate to charity, while some say that one may give up to one-third. The Chasam Sofer says that one shouldn't leave anything to tzedakah because all of the money should go to the children. He only makes an exception in a case where there are no sons and one wants to leave some of his assets to tzedakah as a zechus because he is not leaving behind any sons. Otherwise, he says that one shouldn't do that. So, there is a wide array of opinions on this matter.
Question: Does it make a difference if the reason he wants to give to tzedakah is that he wants to help that charity or because he wants to deny his children the money?
Answer: Most Poskim who discuss the question of tzedakah don't get into that but there are some who explicitly say that the whole dispensation to give a significant amount to tzedakah at the time of death is only when he is doing it l'shem tzedakah; however, if he is doing it specifically to disinherit the rightful heirs, it should not be done. | 
