diff --git a/delete.go b/delete.go
index dd3041f8722..682101ccad8 100644
--- a/delete.go
+++ b/delete.go
@@ -14,10 +14,10 @@ import (
 )
 
 func killContainer(container *libcontainer.Container) error {
-	_ = container.Signal(unix.SIGKILL, false)
+	_ = container.Signal(unix.SIGKILL)
 	for i := 0; i < 100; i++ {
 		time.Sleep(100 * time.Millisecond)
-		if err := container.Signal(unix.Signal(0), false); err != nil {
+		if err := container.Signal(unix.Signal(0)); err != nil {
 			destroy(container)
 			return nil
 		}
diff --git a/kill.go b/kill.go
index e5b13b1230d..ac1e47a5b19 100644
--- a/kill.go
+++ b/kill.go
@@ -1,10 +1,12 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"strconv"
 	"strings"
 
+	"github.com/opencontainers/runc/libcontainer"
 	"github.com/urfave/cli"
 	"golang.org/x/sys/unix"
 )
@@ -24,8 +26,9 @@ signal to the init process of the "ubuntu01" container:
        # runc kill ubuntu01 KILL`,
 	Flags: []cli.Flag{
 		cli.BoolFlag{
-			Name:  "all, a",
-			Usage: "send the specified signal to all processes inside the container",
+			Name:   "all, a",
+			Usage:  "(obsoleted, do not use)",
+			Hidden: true,
 		},
 	},
 	Action: func(context *cli.Context) error {
@@ -49,7 +52,11 @@ signal to the init process of the "ubuntu01" container:
 		if err != nil {
 			return err
 		}
-		return container.Signal(signal, context.Bool("all"))
+		err = container.Signal(signal)
+		if errors.Is(err, libcontainer.ErrNotRunning) && context.Bool("all") {
+			err = nil
+		}
+		return err
 	},
 }
 
diff --git a/libcontainer/container_linux.go b/libcontainer/container_linux.go
index 27ba0597d7e..6f0ccd9a21f 100644
--- a/libcontainer/container_linux.go
+++ b/libcontainer/container_linux.go
@@ -357,32 +357,18 @@ func (c *Container) start(process *Process) (retErr error) {
 	return nil
 }
 
-// Signal sends a specified signal to container's init, or, if all is true,
-// true, to all container's processes (as determined by container's cgroup).
+// Signal sends a specified signal to container's init.
 //
-// Note all=true is implied when s is SIGKILL and the container does not have
-// its own PID namespace. In this scenario, the libcontainer user may be required
-// to implement a proper child reaper.
-func (c *Container) Signal(s os.Signal, all bool) error {
+// When s is SIGKILL and the container does not have its own PID namespace, all
+// the container's processes are killed. In this scenario, the libcontainer
+// user may be required to implement a proper child reaper.
+func (c *Container) Signal(s os.Signal) error {
 	c.m.Lock()
 	defer c.m.Unlock()
 	status, err := c.currentStatus()
 	if err != nil {
 		return err
 	}
-	if all {
-		sig, ok := s.(unix.Signal)
-		if !ok {
-			return errors.New("unsupported signal type")
-		}
-		if status == Stopped && !c.cgroupManager.Exists() {
-			// Avoid calling signalAllProcesses which may print
-			// a warning trying to freeze a non-existing cgroup.
-			return nil
-		}
-		return c.ignoreCgroupError(signalAllProcesses(c.cgroupManager, sig))
-	}
-
 	// To avoid a PID reuse attack, don't kill non-running container.
 	switch status {
 	case Running, Created, Paused:
diff --git a/libcontainer/integration/exec_test.go b/libcontainer/integration/exec_test.go
index e1ee5f7b4ed..03d16639067 100644
--- a/libcontainer/integration/exec_test.go
+++ b/libcontainer/integration/exec_test.go
@@ -1400,7 +1400,7 @@ func testPidnsInitKill(t *testing.T, config *configs.Config) {
 	ok(t, err)
 
 	// Kill the container.
-	err = container.Signal(syscall.SIGKILL, false)
+	err = container.Signal(syscall.SIGKILL)
 	ok(t, err)
 	_, err = process1.Wait()
 	if err == nil {
diff --git a/man/runc-kill.8.md b/man/runc-kill.8.md
index 684666396bd..0d4cb68e2ff 100644
--- a/man/runc-kill.8.md
+++ b/man/runc-kill.8.md
@@ -4,7 +4,7 @@
 **runc-kill** - send a specified signal to container
 
 # SYNOPSIS
-**runc kill** [**--all**|**-a**] _container-id_ [_signal_]
+**runc kill** _container-id_ [_signal_]
 
 # DESCRIPTION
 
@@ -15,15 +15,6 @@ A different signal can be specified either by its name (with or without the
 **SIG** prefix), or its numeric value. Use **kill**(1) with **-l** option
 to list available signals.
 
-# OPTIONS
-**--all**|**-a**
-: Send the signal to all processes inside the container, rather than
-the container's init only. This option is implied when the _signal_ is **KILL**
-and the container does not have its own PID namespace.
-
-: When this option is set, no error is returned if the container is stopped
-or does not exist.
-
 # EXAMPLES
 
 The following will send a **KILL** signal to the init process of the
diff --git a/tests/integration/kill.bats b/tests/integration/kill.bats
index 2a78778bdee..8f2f4a7b4e8 100644
--- a/tests/integration/kill.bats
+++ b/tests/integration/kill.bats
@@ -22,7 +22,12 @@ function teardown() {
 	[ "$status" -eq 0 ]
 	wait_for_container 10 1 test_busybox stopped
 
-	# we should ensure kill work after the container stopped
+	# Check that kill errors on a stopped container.
+	runc kill test_busybox 0
+	[ "$status" -ne 0 ]
+	[[ "$output" == *"container not running"* ]]
+
+	# Check that -a (now obsoleted) makes kill return no error for a stopped container.
 	runc kill -a test_busybox 0
 	[ "$status" -eq 0 ]
 
@@ -31,7 +36,7 @@ function teardown() {
 }
 
 # This is roughly the same as TestPIDHostInitProcessWait in libcontainer/integration.
-@test "kill --all KILL [host pidns]" {
+@test "kill KILL [host pidns]" {
 	# kill -a currently requires cgroup freezer.
 	requires cgroups_freezer
 
@@ -65,7 +70,7 @@ function teardown() {
 		kill -0 "$p"
 	done
 
-	runc kill -a test_busybox KILL
+	runc kill test_busybox KILL
 	[ "$status" -eq 0 ]
 	wait_for_container 10 1 test_busybox stopped