Ferroelectricity has been predicted in two-dimensional Group-Va elemental materials and confirmed in high-quality Bi monolayers by a recent experiment. The origin of such elemental ferroelectricity is related to the spontaneous lattice distortion with atomic layer buckling. A surprising observation in experiment is the abundance of charged 180° head-to-head/tail-to-tail domain walls, distinct from conventional ferroelectrics, where the naturally occurring ferroelectric domain walls are mostly charge neutral. Here, we clarify the origin of this phenomenon. We find that distinct from conventional ferroelectrics, in such single-element ferroelectric monolayers, it is the strain energy rather than the electrostatic energy that dominates the energetics. This leads to intrinsically stable 180° charged domain walls. The orbital interaction and the lone-pair activation mechanism play a key role in this picture. We further predict and confirm experimentally that the most stable domain wall type changes from charged to neutral ones under small applied strain. Our work reveals a mechanism to generate polarization and stabilize intrinsic charged domain walls, which will shed light on potential applications of ferroelectronics based on charged domain walls.