We study electrostatic mechanisms of the destabilization of highly asymmetric electrolytes. For this purpose, we perform primitive model Monte Carlo simulations of charged macroions immersed in multivalent salt solution. At low salt concentration, the macroion effective charge is reduced due to multivalent counterion adsorption. At high salt concentrations, the macroions become overcharged so that their apparent charge has the opposite sign to the stoichiometric one. The inverted charge increases to a saturation value upon further increase of the salinity. The system remains stable at low as well as at very high salt concentrations. In the intermediate region, close to the macroion isoelectric point, we observe macroion aggregation. The obtained phase behavior closely resembles polyelectrolyte-induced instability of colloidal dispersions.