[SPOILERS] How were vikings do dumb in 4x20 ?

I mean even if Ecbert was a legitimate king and he could legitimize their settlement, that all falls down if he dies because the new king will be able to reverse that decision, so why would they let him kill himself ? Also they said "saxons will be mad if they found out we killed their king", do they really think that saxons would believe them that Ecbert just killed himself and not suspect that they killed him and lied about suicide ? Were they really that dumb or just poor writing ?