The decision by two major utilities to terminate their contracts to buy power from the proposed $2.5 billion wind farm in Cape Cod waters could kill the project, according to supporters and detractors of the plan. National Grid and NStar - a subsidiary of Northeast Utilities - said Tuesday they ended their contracts because Cape Wind missed a Dec. 31 deadline set out in the 2012 deals to obtain financing and begin construction, and had chosen not to put up financial collateral to extend the deadline.