Grant Ward did not just kill his brother; Ward killed a [fictional] United States Senator.
Grant Ward already has a huge list of victims that would be a series of long murder and treason trials. Kidnapping and murdering a United States Senator has its own enhanced punishments in Federal Court. 
Federal law states that anyone who kidnaps and kills a United States Senator (or member of Congress or the Executive Branch) shall be punished by death or imprisonment up to life. 18 USCS § 351(b)(2).
Grant Ward beat Senator Ward, killed at least two guards, and tortured the Senator with the forced digging up of a buried well, followed by threats to throw the Senator down the well.
Federal Prosecutors could prove beyond a reasonable doubt that Grant Ward killed his brother with malice aforethought that included kidnapping and torture. 18 USCS § 1111.
There is very little a defense attorney could do for Grant Ward, other than try plea-bargaining for life in prison over the death penalty. Given the number of people Ward killed (at least five SHIELD Agents, four soldiers, two guards, and a Senator) plus the charges for treason by working with HYDRA, it is hard to imagine a Prosecutor even entertaining the idea of a plea; the literal bodies of evidence should end a trial with the death penalty for Grant Ward.
