Inter were sunk by a 70th minute goal from their former forward Goran Pandev, who volleyed home from a rebound after Miguel Veloso’s long-range shot was tipped onto the bar by Samir Handanovic.
The visitors were gifted a late chance for an equaliser when Nicolas Burdisso was harshly ruled to have handled but Candreva’s weak penalty was saved by Eugenio Lamanna.
Inter’s misery was compounded with a red card for midfielder Geoffrey Kondogbia for dissent.
The defeat left them seventh in the table, outside the Europa League places. Fiorentina pulled level with them on 56 points after Federico Bernardeschi scored in the fourth minute to stoppage time to give them a 2-2 draw at Sassuolo.
Palermo earned a stay of execution with a 2-0 win over Fiorentina last week but there was no escape this time even after Edoardo Goldaniga scored a late equaliser to cancel out Sergio Pellissier’s penalty.
The Sicilians, who are one off the bottom with 20 points, were left 12 adrift of safety with three games to play after Empoli beat Bologna 3-1.
Keita Balde Diao gave Lazio a second-minute lead but the game turned when Sampdoria defender Milan Skriniar fouled the Senegalese, was sent off and conceded a penalty, converted by Ciro Immobile, in the 20th minute.
Although Karol Linetty pulled one back, further goals from Wesley Hoedt, a Felipe Anderson penalty and Stefan de Vrij made it 5-1 at halftime. Senad Lulic and Immobile made it 7-1 with 20 minutes left before Fabio Quagliarella pulled two back, one from a penalty.