Self Destruct in geth solidity - solidity

Question

I found SelfDestruct problem in geth
I didn't call SelfDestruct on my smart contract
but, after restart my server PC which implements geth, my contract suddenly selfdestruct.
I have no sense about this kind of problem.
is there any condition of selfdestruct which I didn't know?
pragma solidity^0.4.19;
contract Dan_v1_1 {
struct DanInfo {
string file_path;
string kor_name;
string personal_key;
string eng_name;
uint8 gradeOfpoomDan;
string danCertificate;
uint[] regDateHistory;
bool flag;
}
mapping (string => DanInfo) danInfo;
mapping (uint => string) indexOfDanInfo;
uint private danInfoCount = 0;
address contractOwner;
function Dan_v1_1() public{
contractOwner = msg.sender;
}
function setInitDanInfo (string file_path,string kor_name, string personal_key,string eng_name,
uint8 gradeOfpoomDan,string danCertificate, uint []regDateHistory, bool flag)
public{
require (keccak256(danInfo[personal_key].personal_key) != keccak256(personal_key));
danInfo[personal_key] = DanInfo(file_path,kor_name, personal_key,eng_name,gradeOfpoomDan,danCertificate,regDateHistory,flag);
indexOfDanInfo[danInfoCount++] = personal_key;
}
function getDanInfoCount () public constant returns(uint) {
return danInfoCount;
}
function getDanInfo (string personal_key) public constant returns(string,string, string, string, uint8, string, uint[], bool) {
DanInfo memory temp = danInfo[personal_key];
return(temp.file_path,temp.kor_name, temp.personal_key, temp.eng_name, temp.gradeOfpoomDan, temp.danCertificate, temp.regDateHistory, temp.flag);
}
function killContract() public{
if(contractOwner == msg.sender) {
selfdestruct(contractOwner);
}
}
}