Stars with ∼8-10 M evolve to form a strongly degenerate ONeMg core. When the core mass becomes close to the Chandrasekhar mass, the core undergoes electron captures on 24Mg and 20Ne that induce an electron-capture supernova (ECSN). In order to clarify whether the ECSN leads to a collapse or thermonuclear explosion, we calculate the evolution of an 8.4 M star from the main sequence until the oxygen ignition in the ONeMg core. We apply the latest electron-capture rate on 20Ne, including the second forbidden transition, and investigate how the location of the oxygen ignition (center or off-center) and the Ye distribution depend on the input physics and the treatment of the semiconvection and convection. The central density when the oxygen deflagration is initiated, c,def, can be significantly higher than that of the oxygen ignition thanks to the convection, and we estimate . We perform two-dimensional simulations of the flame propagation to examine how the final fate of the ONeMg core depends on the Ye distribution and c,def. We find that the deflagration starting from leads to a collapse (thermonuclear explosion). Since our estimate of c,def exceeds this critical value, the ONeMg core is likely to collapse, although further studies of the convection and semiconvection before the deflagration are important.