There are five elements of justifiable defense, namely
1. Causal condition: violation of reality;
2. Time conditions: the infringement is going on;
3. Subjective conditions: having a sense of defense;
4. Object condition: defense against the infringer;
5. Limit conditions: the necessary limit is not obviously exceeded.
Judging from this case, Party B has no murder weapon, while Party A has a knife. At the same time, the limit of Party A's defense is only to subdue Party B, but Party A stabbed Party B several times, causing death. Has been suspected of intentional homicide, not excessive defense.