The burden of proof
لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لَادَّعَى نَاسٌ دِمَاءَ رِجَالٍ وَأَمْوَالَهُمْ، وَلَكِنَّ الْيَمِينَ عَلَى الْمُدَّعَى عَلَيْهِ
Law yu'ta-n-nasu bi-da'wahum la-dda'a nasun dima'a rijalin wa amwalahum, wa lakinna-l-yamina 'ala-l-mudda'a 'alayh.
"If people were given whatever they claimed, some would claim the lives and the wealth of others. But the oath is on the one the claim is made against."
Narrated by Ibn Abbas. Sahih Muslim 1711, Sahih al-Bukhari 4552 · Nawawi's Forty · #33
A short reflection
An accusation is not evidence. A claim on someone's life or property has to be proved, and the one who denies it is asked only for an oath.